feat(watch): add config option to disable linked worktree discovery

[rusel95]

Problem

Fixes #211. grepai watch in a main worktree unconditionally discovers all linked git worktrees, auto-initializes them (creates .grepai/, edits their .gitignore) and watches them alongside the main project. There is no way to opt out.

For workflows that create many short-lived worktrees — AI-agent sessions, git worktree-based review checkouts — this multiplies indexing work, embedding calls and resident memory by the worktree count. The #211 author tried five separate workarounds (ignore patterns, per-worktree configs, killing sub-watchers…) — none work, because discovery is a separate mechanism from file scanning and ignores all of them.

Change

One new key in .grepai/config.yaml, read in the main worktree:

watch:
  discover_worktrees: false   # default: true

• Backward compatible by construction: the field is a *bool (omitempty), so existing configs — and configs rewritten by the watcher — keep the historical behavior (enabled) unless the user explicitly opts out. WorktreeDiscoveryEnabled() encapsulates the default.
• Single choke point: the guard lives inside discoverWorktreesForWatch, which is the sole entry to discovery, so the initial scan, the TUI path and the supervisor's periodic reconcile all respect it.
• Live toggle: the config is re-read on every discovery pass (3s reconcile), so flipping the option takes effect within seconds without restarting the watcher; already-watched worktrees are released on the next pass.
• Fails closed: if the config can't be loaded (e.g. a torn write while the user edits it between two reconcile passes), discovery is skipped for that pass and a warning is logged — a broken config can never silently re-enable the side-effectful auto-init the user opted out of.

Tests

• main worktree discovers + auto-inits a linked worktree (baseline, key absent)
• discover_worktrees: false → no discovery, no auto-init side effects
• linked worktree never discovers siblings regardless of the option
• unreadable/torn config → fail closed, no discovery, no auto-init
• WorktreeDiscoveryEnabled() accessor: nil/true/false table test
• YAML round-trip: explicit false survives config load/save

Docs

configuration.md and watch-guide.md now list the key; git-worktrees.md gains a "Disabling Worktree Discovery" section and two troubleshooting rows (discovery not triggering; too many worktrees indexed).

🤖 Generated with Claude Code