test: use os-assigned ports for test servers

[KyleJune]

Summary

Test servers no longer bind fixed port 8100, so multiple test runners (e.g. parallel AI agents) can run simultaneously without "address already in use" failures. Note that leaving DENO_SERVE_ADDRESS unset is not an option: Deno.serve then defaults to fixed port 8000, which would collide with the dev server instead.

Changes

• Spawn-style main.test.ts tests (example, 4 templates, blog tutorial) now inject DENO_SERVE_ADDRESS=tcp:127.0.0.1:0 into the spawned child so the OS assigns a free port, parse the actual URL from the child's Listening on line, and fetch that URL instead of a hardcoded http://localhost:8100/. The Windows-specific exact-line assertion existed only to check the fixed port and is removed.
• Removed DENO_SERVE_ADDRESS from all .env.test files.
• Synced docs: docs/getting-started.md, docs/tutorials/blog.md (embedded test snippet matches the real file), template/example/tutorial READMEs, and the stale port example in src/utils/global-jsdom.ts (its 8000 fallback behavior is unchanged).

Testing

All project suites pass: src (19 tests / 266 steps), example, all 4 templates, and the blog tutorial. Parallel-safety was proven directly: two concurrent runs of the same spawn test bound different OS-assigned ports (42695 / 34611) and both passed; nothing is left listening on 8100 afterward.

🤖 Generated with Claude Code

[KyleJune]

docs: recommend dot reporter for test runs — AGENTS.md Testing section now says to use --reporter=dot when running all tests or any run that doesn't need a test's console output (detail printed only for failures); drop it only when debugging with logs. Companion to udibo/udibo#59.