devcontainer: bridge host known_hosts so push works under the CLI (#125)

Mirrors the existing host-gitconfig handling. Without this, git push from
inside a CLI-launched devcontainer fails with "Host key verification
failed" the first time it talks to github.com \xe2\x80\x94 the base image leaves
~/.ssh empty and SSH refuses unknown fingerprints by default.

The fix is the same shape as the gitconfig bridge:
- initialize.sh snapshots ~/.ssh/known_hosts into
  .git-plumbing/host-known-hosts (empty file if absent).
- post-start.sh copies that into $HOME/.ssh/known_hosts only when the
  in-container file is missing or empty, creating ~/.ssh mode 700 first.
- .gitignore covers the new runtime artifact.
- README and CLAUDE.md updated alongside.

VS Code\xe2\x80\x99s Dev Containers extension already bridges known_hosts itself, so
the empty-check naturally lets it win when it\xe2\x80\x99s involved. CI\xe2\x80\x99s
`devcontainer build` never reaches postStart, so the script is a no-op
there.

Author: Syndic

.claude/CLAUDE.md

@@ -62,12 +62,13 @@ the git path symlink).
 The host signs with **SSH** (`gpg.format = ssh`, `commit.gpgsign = true`, no explicit
 `user.signingkey` — `gpg.ssh.defaultKeyCommand` shells out to `ssh-add -L`). That makes two
 things load-bearing inside the container: a usable ssh-agent socket the in-container `git` can
-reach, and the host's `~/.gitconfig`. VS Code's Dev Containers extension supplies both
-automatically — it forwards the host ssh-agent through the VS Code Server's own SSH tunnel (a
-per-user socket published by the server process, *not* Docker Desktop's magic socket — that
-mount isn't even present in extension-launched containers) and copies the host gitconfig
-between `postCreate` and `postStart`. The `devcontainer` CLI does neither. The fix is three
-additive pieces:
+reach, and the host's `~/.gitconfig`. Pushing the resulting signed commit then needs a third —
+the host's `~/.ssh/known_hosts`, or SSH refuses the unknown github.com fingerprint. VS Code's
+Dev Containers extension supplies all three automatically — it forwards the host ssh-agent
+through the VS Code Server's own SSH tunnel (a per-user socket published by the server process,
+*not* Docker Desktop's magic socket — that mount isn't even present in extension-launched
+containers), and bridges the host gitconfig + known_hosts between `postCreate` and `postStart`.
+The `devcontainer` CLI does none of that. The fix is four additive pieces:
 
 - **SSH agent** — `devcontainer.json` binds Docker Desktop's magic socket
   `/run/host-services/ssh-auth.sock` (Desktop's documented mechanism for exposing the host's
@@ -94,6 +95,15 @@ additive pieces:
   reaches postStart, so the file is absent there and the script is a clean no-op. Same
   lifecycle/buildx-safety story as `host-git-common-path` and `host-timezone` — gitignored,
   regenerated every `up`, anchored by the tracked `.git-plumbing/README.md`.
+- **`~/.ssh/known_hosts`** — same shape as the gitconfig copy. `initialize.sh`
+  snapshots the host's `~/.ssh/known_hosts` to `.git-plumbing/host-known-hosts`;
+  `post-start.sh` installs it into `$HOME/.ssh/known_hosts` only if that file
+  is missing or empty (creating `~/.ssh` mode 700 first). Without it, `git
+  push` from inside the CLI-launched container fails with "Host key
+  verification failed" on first contact with github.com — the base image's
+  `~/.ssh` is empty and SSH refuses unknown fingerprints by default. VS Code
+  bridges known_hosts itself, so the empty-check leaves that path alone.
+  Same gitignored / regenerated-every-`up` lifecycle as the gitconfig snapshot.
 
 `gpg.ssh.allowedSignersFile` in the copied gitconfig points to a host-only path that doesn't
 exist in the container. Irrelevant: git only reads it for `git verify-commit`, not for

.devcontainer/.git-plumbing/README.md

@@ -18,6 +18,12 @@ image. The files are gitignored and written on every `devcontainer up` by
   the `devcontainer` CLI path; VS Code's Dev Containers extension copies
   it for itself). See ".devcontainer signed commits under CLI" in
   `.claude/CLAUDE.md`.
+- `host-known-hosts` — snapshot of the host's `~/.ssh/known_hosts`.
+  `post-start.sh` installs it into the container only when
+  `~/.ssh/known_hosts` is empty, so `git push` from inside the CLI-
+  launched container doesn't trip "Host key verification failed" on first
+  contact with github.com. Same VS-Code-wins behaviour as the gitconfig
+  copy.
 
 This directory exists in git (via this README) so the Dockerfile's
 `COPY .devcontainer/.git-plumbing/ …` step always finds a source — buildx

.devcontainer/initialize.sh

@@ -58,6 +58,7 @@ link="$here/.host-git-common"
 pathfile="$here/.git-plumbing/host-git-common-path"
 tzfile="$here/.git-plumbing/host-timezone"
 gitconfigfile="$here/.git-plumbing/host-gitconfig"
+knownhostsfile="$here/.git-plumbing/host-known-hosts"
 
 # The .git-plumbing dir is tracked (via its README), so it normally exists
 # already; mkdir -p covers stray cases like a manual deletion without
@@ -119,6 +120,19 @@ else
   : >"$gitconfigfile"
 fi
 
+# Snapshot host ~/.ssh/known_hosts the same way. Without it, `git push` from
+# inside a CLI-launched container fails with "Host key verification failed"
+# the first time it talks to github.com — the base image's $HOME/.ssh is
+# empty and SSH refuses unknown fingerprints by default. The host's
+# known_hosts is the right trust set to carry (same flavor as the gitconfig
+# — both are the user's existing trust state, neither secret). Empty file
+# if absent so the `[ -s ... ]` guard in post-start.sh stays a clean no-op.
+if [ -r "$HOME/.ssh/known_hosts" ]; then
+  cp "$HOME/.ssh/known_hosts" "$knownhostsfile"
+else
+  : >"$knownhostsfile"
+fi
+
 # Pre-create the magic ssh-agent socket placeholder on hosts where Docker
 # Desktop isn't intercepting it (CI runners, plain Docker on Linux). Docker
 # Desktop auto-forwards the host ssh-agent at /run/host-services/ssh-auth.sock

.devcontainer/post-start.sh

@@ -12,6 +12,22 @@ if [ ! -s "$HOME/.gitconfig" ] && [ -s "$src" ]; then
   cp "$src" "$HOME/.gitconfig"
 fi
 
+# Install host ~/.ssh/known_hosts the same way. Lets `git push` from inside
+# the CLI-launched container succeed first try; without it the user hits
+# "Host key verification failed" because the base image has no known_hosts
+# and SSH refuses unknown fingerprints by default. VS Code's Dev Containers
+# extension already bridges known_hosts when it's involved, so the empty-
+# check leaves that path alone.
+known_src="$here/.git-plumbing/host-known-hosts"
+known_dst="$HOME/.ssh/known_hosts"
+if [ ! -s "$known_dst" ] && [ -s "$known_src" ]; then
+  # ~/.ssh must be mode 700 or SSH ignores it; known_hosts at 644 is fine.
+  mkdir -p "$HOME/.ssh"
+  chmod 700 "$HOME/.ssh"
+  cp "$known_src" "$known_dst"
+  chmod 644 "$known_dst"
+fi
+
 # Docker Desktop bind-mounts the magic ssh-agent socket root-owned mode 660,
 # so the non-root remoteUser can't connect until we re-own it.
 sock=/run/host-services/ssh-auth.sock

.gitignore

@@ -64,6 +64,7 @@ target/
 /.devcontainer/.git-plumbing/host-git-common-path
 /.devcontainer/.git-plumbing/host-timezone
 /.devcontainer/.git-plumbing/host-gitconfig
+/.devcontainer/.git-plumbing/host-known-hosts
 
 # ── Claude Code ───────────────────────────────────────────────────────────────────────────────────
 # settings.local.json holds per-developer overrides — never shared