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>

.github/CONTRIBUTING.md

@@ -55,7 +55,7 @@ npm run test:all       # literally everything, environmental failures included
 
 Expect `test:browser`/`test:mobile`/`test:perf` to fail where the machine cannot provide what they need; read that as "not runnable here", not as a regression. `config/test-suites.ts` holds the globs, and both configs derive from it, so the exclusions and those runners cannot drift apart.
 
-If you add a test that binds a port, pick a unique one at 3150 or above (search the repo for `const PORT =` first). Never 3000.
+If you add a test that binds a port, bind port 0 (`new WebServer(0, …)` + `server.boundPort`, or `listen({ port: 0 })` + `address().port`), or use `app.inject()` when no socket is needed; `test/test-ports-guard.test.ts` fails a `WebServer` built on any other port. Mobile tests (`test/mobile/**`, via `createTestServer(PORT)`) keep the fixed-port convention in `test/mobile/README.md` for now, because that helper caches servers by port. Never 3000.
 
 Tests are tmux-safe by design: under vitest, the tmux layer becomes an in-memory mock, so tests cannot touch real sessions.