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>

AGENTS.md

@@ -12,6 +12,6 @@ Quick pointers:
 
 - Type check: `tsc --noEmit` · Lint: `npm run lint` · Format: `npm run format:check`
 - Tests: `npm test` (the CI gate, safe to run bare) or `npm test -- test/<file>.test.ts` for one file
-- Route tests use `app.inject()`; new tests needing ports must pick a unique `const PORT =`
+- Route tests use `app.inject()`; new tests needing a socket bind port 0 (`new WebServer(0, …)` + `boundPort`), except mobile tests, which keep `createTestServer(PORT)` for now
 - Branch off `master` for all work; Conventional Commit-style messages (`fix(mobile): ...`)
 - Never commit secrets or local state from `~/.codeman/`