implement builtin.claude per D-041

Tier-0 stub flipped to the real Claude pane. Primary-per-repo
semantics: session name is `clide-claude-<hash>` where <hash> is
FNV-1a over the workspace root path, so reopening clide attaches
to the same `claude` process through `tmux new-session -A`.

Primary has no close button — closing the tab disposes the widget
but deliberately does NOT call pane.close, so the tmux session
survives until the user actually exits claude or clide is shut
down. Secondary sessions (spawned via the claude.new-secondary
command registered here, UI wiring lands next) close normally and
the pane.close cascades into tmux kill-session.

Graceful fallback when tmux isn't on PATH: retries the spawn with
argv=['claude'], surfaces "no-tmux · fresh every launch" in the
header subtitle so the user knows persistence is off.

Session-naming unit tests cover determinism + uniqueness per repo.
Full app suite: 174 tests passing.

Co-Authored-By: Claude <noreply@anthropic.com>

CHANGELOG.md

@@ -18,6 +18,18 @@ heading, and (b) bumping `project.yaml` `version:` in the same commit.
 
 ### Added
 
+- `builtin.claude` — Tier-1 stub upgraded to the real Claude pane per
+  D-041. Contributes a primary `Claude` tab in the workspace slot that
+  spawns `tmux new-session -A -s clide-claude-<hash> -- claude` via
+  IPC `pane.spawn`, with `<hash>` derived from the git root path so
+  reopening the app re-attaches to the running conversation. Primary
+  has no close affordance; closing the tab doesn't kill the session.
+  Command `claude.new-secondary` is registered for the palette wiring
+  that's coming next. If tmux isn't on PATH, falls back to spawning
+  `claude` directly and surfaces "no-tmux · fresh every launch" in
+  the header subtitle. Accompanied by D-041 in
+  [`decisions/architecture.md`](decisions/architecture.md#d-041-claude-panes-one-primary-per-repo-tmux-backed).
+
 - `builtin.files` — workspace filesystem panel in the sidebar. Lazy
   tree rooted at the git root, expand/collapse, click-to-open plumbed
   to a future `editor.open` command. Backed by a new daemon-side