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>

docs/wiki/Contributing.md

@@ -61,8 +61,11 @@ benchmarks for an otherwise idle machine). Expect those to fail where the machin
 provide what they need; that means "not runnable here", not a regression.
 
 Tests are tmux-safe by design: under vitest the tmux layer becomes an in-memory mock, so
-tests cannot touch real sessions. If you add a test that binds a port, pick a unique one at
-3150 or above, and never 3000.
+tests cannot touch real sessions. 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. Never 3000. Mobile tests (`test/mobile/**`,
+via `createTestServer(PORT)`) keep the fixed ports in `test/mobile/README.md` for now,
+because that helper caches servers by port.
 
 ## Finding your way around