feat(cli): add specify self check and self upgrade stub (#2316)

* feat(cli): add specify self check and self upgrade stub (#2282)

Introduce a new `specify self` Typer sub-app with two subcommands.

`specify self check` performs a read-only lookup against the GitHub Releases
API, compares the installed version to the latest tag with PEP 440 semantics,
and prints one of four verdicts (newer-available, up-to-date, indeterminate,
graceful-failure). When a newer stable release is available, the output
includes a copy-pasteable `uv tool install --force --from git+...@<tag>`
reinstall command. `GH_TOKEN` / `GITHUB_TOKEN` is attached as a bearer
credential when set so users behind shared IPs escape the anonymous 60/hour
rate limit.

`specify self upgrade` is a documented non-destructive stub in this release:
three-line guidance output, exit 0, no outbound call, no install-method
detection. The real destructive implementation is planned as follow-up work.

Failure categorization is a fixed three-entry enum (offline or timeout,
rate limited, HTTP <code>). Anything outside those three categories
propagates as a non-zero exit so bugs surface instead of being silently
swallowed. No machine-readable output, no retries, no caching in this
release — see issue #2282 discussion.

Tests mock `urllib.request.urlopen`; the suite performs zero real network
calls. Full regression suite: 1586 passed.

* fix(cli): disable Rich highlight for deterministic output

Rich's default `highlight=True` applies ANSI color to detected patterns
(integers, version strings, paths) whenever stdout is deemed a TTY.
This caused intermittent failures in existing pytest assertions in
tests/test_cli_version.py and tests/test_extensions.py::TestExtensionRemoveCLI
that compare plain-text output without passing through `strip_ansi()`.

Setting `Console(highlight=False)` globally makes all CLI output
deterministic and fixes the flake without modifying the affected tests.
The numeric cyan highlighting was not a documented part of the CLI
visual contract.

* fix: address copilot review feedback

* fix: tighten self-check token handling

* fix: align self-check helpers and script metadata

* fix: harden self-check version handling

* fix: guard self-check failure rendering