fix(lanes): removing a lane deletes its record only, never the worktree

A lane's `remove` tore down what the lane pointed at: `git worktree remove
--force`, a branch delete, and a drop of the lane's own database. One
mis-click on a dashboard row cost a whole branch's unpushed code — and the
row is the only thing AgentHub actually owns. `remove` now stops the run and
the stack, clears AgentHub's own runtime bookkeeping under
`LANES_ROOT/.state/lane<slot>/`, and deletes the row. The working tree, its
branch, git's worktree records and the database stay exactly where they are,
for the user to delete by hand when they mean to.

That also fixes #28. Removal used to route through `assertDestroyable`, whose
third check demands the cwd appear in `git worktree list`; a managed lane
whose directory git no longer listed answered `ENOTWORKTREE` ("lane is not
listed as a worktree in …") and could never be dropped from the dashboard at
all. Removal no longer asks git anything, so no preflight blocker stops it,
and the unpushed-commits force-gate now applies to `reset` alone — nothing
`remove` does can discard a commit.

The UI follows: one label for both lane kinds ("Delete lane record"), copy
that says what survives, and no Force checkbox on removal.
`removeWorktree`/`removeLaneData` remain in place as guarded primitives; no
route calls them any more.

server/__tests__/lanes-cli.test.js

@@ -546,13 +546,14 @@ describe("agenthub lanes destructive lifecycle", () => {
     assert.match(loser.stdout, /Current preflight:/);
   });
 
-  it("removes a managed lane's worktree and directory with --yes", async () => {
+  it("removes a managed lane's record with --yes, leaving its directory on disk", async () => {
     const lane = await ownLane("CLI remove managed");
     assert.equal(fs.existsSync(lane.cwd), true);
     const r = await cli(["lanes", "remove", String(lane.id), "--yes"], SOURCE_REPO);
     assert.equal(r.status, 0, r.stderr);
     assert.match(r.stdout, new RegExp(`Removed lane #${lane.id}`));
-    assert.equal(fs.existsSync(lane.cwd), false);
+    assert.match(r.stdout, /left for you to delete yourself/);
+    assert.equal(fs.existsSync(lane.cwd), true);
     assert.equal((await get(`/api/lanes/${lane.id}`)).status, 404);
   });