fix(tests): #556 landing fixes

Move test/sse-tile-grid-filter.test.ts to an ephemeral port. The release
added it with #561 on fixed port 3287, after #556 was cut, so it is not on
the guard's LEGACY_FIXED_PORT_FILES and test/test-ports-guard.test.ts failed
on the merged tree. It now builds new WebServer(0, ...) and its url() helper
reads server.boundPort (only ever called inside tests, after beforeAll).
Converting it is preferred over listing it, since the legacy list is
shrink-only.

Update the five docs that still told contributors to pick a unique fixed
port, which the new guard now rejects for any WebServer test: CLAUDE.md
(Adding Features and Testing), AGENTS.md, .github/CONTRIBUTING.md and the
wiki's Contributing page (mirrored to the public GitHub wiki). They now say
to bind port 0 and read boundPort (or address().port for a raw server), and
note that the mobile suite keeps its fixed ports for now, because
test/mobile/helpers/server.ts caches servers by port, so createTestServer(0)
from two callers would share one server.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

test/sse-tile-grid-filter.test.ts

@@ -1,6 +1,6 @@
 /**
  * @fileoverview The server's side of the tile grid's SSE filter (live server,
- * multi-user mode, port 3287).
+ * multi-user mode, ephemeral port).
  *
  * While tiles own the terminal the page subscribes with TILE_GRID_SSE_FILTER
  * (constants.js), an id that names no session, so the server sends it no
@@ -31,8 +31,7 @@ import { createUser, invalidateUsersCache } from '../src/user-store.js';
 
 vi.spyOn(TmuxManager, 'isTmuxAvailable').mockReturnValue(true);
 
-const PORT = 3287;
-const url = (p: string) => `http://localhost:${PORT}${p}`;
+const url = (p: string) => `http://localhost:${server.boundPort}${p}`;
 const basic = (u: string, p: string) => 'Basic ' + Buffer.from(`${u}:${p}`).toString('base64');
 const alice = { Authorization: basic('alice', 'alicepass1') };
 
@@ -124,7 +123,7 @@ beforeAll(async () => {
   await createUser({ username: 'root', role: 'admin', password: 'rootpass123' });
   await createUser({ username: 'alice', role: 'user', password: 'alicepass1' });
   await createUser({ username: 'bob', role: 'user', password: 'bobpass1234' });
-  server = new WebServer(PORT, false, true);
+  server = new WebServer(0, false, true);
   await server.start();
 });