feat(lanes): stage/undo one hunk at a time, sticky review file headers

Review and the single-file view gain per-hunk Stage and Undo
(stage-hunk / discard-hunk: the hunk is named by its @@ line and
re-matched against a fresh diff, then git apply --cached / -R; undo
needs confirm: true and two clicks). Each review file header stays
pinned while its diff scrolls and carries the file's own
stage/unstage/discard.

ARCHITECTURE.md

@@ -369,7 +369,7 @@ graph TD
 | `lib/lane-services.js`    | (A2) `ensureDatabase`/`dropDatabase` call the profile's `db-create`/`db-drop` hooks — AgentHub stays stack-agnostic on purpose. A state-dir marker file tracks whether a slot's database was already created, since a plain `createdb` can't be re-run safely and AgentHub can't assume the hook is idempotent; this is also how `upLane` knows to seed only a freshly-created database. `dropDatabase` asserts the lane is `managed` and that the name being dropped is one this lane's own slot actually derives (itself or its `_test` sibling) before spawning anything |
 | `lib/lane-features.js`    | (B) Per-feature state and archive. `activateFeature` archives the lane's current active feature (if different) and restores the target's saved stage onto the live `lanes` row — the row stays the one live view every other reader already uses. `canonicalizeSlug` is a DELIBERATELY separate rule from `worktree.js:slugify` (drops a leading `feat/`, keeps `[A-Za-z0-9._-]`, does not lowercase) — the two must never be conflated. `clearLane` (`lib/lanes.js`) archives the active feature (if any) before resetting; a lane that never activated one is unaffected |
 | `lib/git-diff.js`         | (#34/#35) Reads a lane's code changes as structured hunks for the Workspace Changes panel — `work` (HEAD vs the working tree, untracked files included), `branch` (`<base>...HEAD`) or `commit` (one sha), at a caller-chosen `context` (default 3, clamped to 5000, which is how the viewer expands a diff towards the whole file). Also the four reads behind History, the whole-file view, quick-open and the file explorer: `listCommits`, `readFileAt` (the `work` revision is realpath-confined to the lane; anything else goes through `git show`), `listPaths` and `listTree` (both `git ls-files`, so `.gitignore` is honoured without a skip-list). Read-only, argv-only through `worktree.js:git()`. An untracked file's patch is SYNTHESIZED from the file itself rather than fetched with `git diff --no-index`, which exits non-zero on a difference and would read as a failure. Three caps (300 files, 200 KB per file, 1 MB per full read) keep a lane mid-build from shipping megabytes of patch into a browser; a cap that bit sets `truncated` |
-| `lib/git-ops.js`          | (#34) The git WRITES a lane offers: `fetch`, `pull`, `push`, `stage`, `unstage`, `discard`, `commit` (everything, or `only:"staged"`), `switch`, `branch-create` (never switches) and `branch-delete` (`-d`, refusing `main`/`master`/`development`/`develop`, the current branch and one another worktree holds). A caller-supplied path is validated against `git status` before it reaches an argv, so only a path git already reports as changed is reachable; `discard` — the one irreversible action — additionally requires `confirm: true`. Deliberately the boring half of git — no merge, rebase, reset or stash, because a dashboard button is the wrong place to start an operation whose recovery needs a terminal (`reset` stays behind worktree.js's three-check destroy guard). `pull` is ALWAYS `--ff-only` and `switch` refuses a dirty tree, so neither can invent a merge commit or a conflict underneath a running agent; `push` pushes `HEAD`, never a branch name from the request. Network actions carry a 60s timeout |
+| `lib/git-ops.js`          | (#34) The git WRITES a lane offers: `fetch`, `pull`, `push`, `stage`, `unstage`, `discard`, `stage-hunk`/`discard-hunk` (one hunk, matched by its `@@` line against a fresh diff; `discard-hunk` needs `confirm:true`), `commit` (everything, or `only:"staged"`), `switch`, `branch-create` (never switches) and `branch-delete` (`-d`, refusing `main`/`master`/`development`/`develop`, the current branch and one another worktree holds). A caller-supplied path is validated against `git status` before it reaches an argv, so only a path git already reports as changed is reachable; `discard` — the one irreversible action — additionally requires `confirm: true`. Deliberately the boring half of git — no merge, rebase, reset or stash, because a dashboard button is the wrong place to start an operation whose recovery needs a terminal (`reset` stays behind worktree.js's three-check destroy guard). `pull` is ALWAYS `--ff-only` and `switch` refuses a dirty tree, so neither can invent a merge commit or a conflict underneath a running agent; `push` pushes `HEAD`, never a branch name from the request. Network actions carry a 60s timeout |
 | `lib/proof.js`            | (C) Proof gallery: lists/serves/deletes QC screenshots grouped by feature slug (Task B's `lane_features`) and phase, under `<lane.cwd>/.playwright-mcp/proof/<slug>/<group>/`. Every path is resolved, realpath'd, and containment-checked before touching `fs` — the entire security surface. `ensureProofLink` ports Shipyard's `ensure_proof_link` (converge a stray clone-root `proof/` onto the canonical dir); never auto-invoked, exposed only as `agenthub lanes proof-link` |
 | `lib/named-lock.js`       | (D) Cross-lane named locks — the OTHER axis from `lib/lane-lock.js`'s per-lane, in-process serialization, deliberately a separate module. `mkdir` is the atomicity primitive (EEXIST decides "already held" in one syscall, never check-then-create). `LOCK_MAX_HOLD` (default 2700s) breaks a stale holder on the next acquire, floored at 300s so the floor — not the configurable default — is the actual safety property: nothing can force-break a live holder by setting the env var low. Single-shot only; the CLI's `agenthub lock acquire` owns the polling loop, keeping the server side non-orchestrating like every other lane primitive |