fix(board): store paths home-relative so shared state works in sandboxes

[dgageot]

The board stores project paths in the user config and card state (RepoPath, Worktree) as absolute paths in ~/.cagent/board/cards.json. When the same state is shared between a host and a Docker sandbox — where the home directory differs but home-relative mounts like ~/src and ~/.cagent are mirrored — those absolute paths don't exist inside the container. The result is that tmux's -c flag silently falls back to $HOME, and agents fail with --worktree requires /home/agent to be inside a git repository.

The fix introduces contractHome/expandHome helpers in pkg/board/paths.go that persist all repo and worktree paths in ~-contracted form and expand them on load relative to the current home. Old absolute-path state files remain loadable; after one run the state is rewritten in the portable form. On top of that, CreateCard now does a fast isGitRepo check before accepting a path, so the failure is a clear error at card-creation time rather than a cryptic tmux/worktree message later. The isGitRepo check also rejects bare repos by inspecting command output.

A second pass hardened the edge cases found in review: contractHome is a no-op when the home directory cannot be determined (avoiding relative-path corruption), null entries in cards.json no longer cause a panic on load, and the isGitRepo logic was tightened. Tests were added for contractHome/expandHome, the store round-trip, and the card-creation guard; the full test suite, build, and linter all pass.

[docker-agent]

Assessment: 🟡 NEEDS ATTENTION

The path-contraction/expansion round-trip (contractHome/expandHome) is correct, the null-entry guard in the store is correct, and the overall approach is sound. Two issues were found in the new isGitRepo pre-check added to CreateCard.

[docker-agent]

[MEDIUM] isGitRepo silently passes when project.Path is empty, running git in the process's cwd

CreateCard calls isGitRepo(a.ctx, project.Path) without first validating that project.Path is non-empty. In Go's os/exec, setting cmd.Dir = "" causes the subprocess to inherit the parent's working directory — so if the board process happens to be running inside a git repository, isGitRepo("") returns true, card creation succeeds, and a card with no valid path is committed to state.

An empty project.Path can arise if expandHome is called on a "" stored value (it returns "" unchanged), or if a caller constructs a Project struct without setting Path.

The companion AddProject already guards against this via normalizeProjectPath, which rejects empty paths explicitly. CreateCard needs a matching guard:

if strings.TrimSpace(project.Path) == "" {
    return nil, errors.New("project path is required")
}

This guard should be placed before the isGitRepo call (line 241).

[docker-agent]

[LOW] Context cancellation or missing git binary produces misleading "not a git repository" error

isGitRepo discards the underlying exec error and returns only a bool. If a.ctx is cancelled (e.g. during app shutdown) or git is not on PATH, the function returns false, and CreateCard reports:

"project path X is not a git repository in this environment; re-add the project here"

…even though the real cause is an execution failure, not an invalid path. This message guides the user to remove and re-add the project, which won't help.

In practice a.ctx is only cancelled at shutdown and git is a hard dependency of this tool, so this is a low-impact UX edge case. Still, distinguishing execution errors from "path is not a repo" would make the error actionable. One option: have isGitRepo return (bool, error) and check errors.Is(err, context.Canceled) before emitting the path-specific message.