fix: stop parsing email domains as maintainer handles

[akshatshahh]

Closes #464.

What: ContextSieve._parse_maintainers_file() ran its handle regex over the whole file, so the domain of every email address became a handle: - Alice Example <alice@realcorp.io> @alice produced ['@realcorp', '@alice', 'Alice Example'], scored 0.95 (EXPLICIT_FILE), auto-accepted, and the phantom handle was later rendered into CODEOWNERS.

Fix: the parser now strips email addresses before matching handles and requires the @ not to be glued to a word character (covers address-like leftovers such as user@localhost). The issue's example now yields ['@alice']. Display names are no longer emitted as maintainers either: a bare name is not a GitHub handle and GitHub rejects it as a code owner.

Tests: 6 new tests in tests/darnit/context/test_context_sieve.py — the issue's exact example, name+email with no handle, a bare email address, plain @mentions (behavior preserved), a handle glued directly after an email, and an end-to-end detect('maintainers') check. 5 of the 6 fail on the pre-fix code. Full context + remediation suites pass (346 passed), ruff check clean.

AI assistance disclosure (per AI_POLICY.md): this PR was prepared with Muse (muse.ai), an AI coding assistant, which drafted the parser fix in packages/darnit/src/darnit/context/sieve.py and the 6 new tests in tests/darnit/context/test_context_sieve.py. I reviewed every line of the change, ran the full test suite, and stand behind it.

[mlieberman85]

Thanks, the parsing fix looks right. I checked the issue's example plus user@localhost @erin, [@bob](url) and (alice@corp.io), and all parse correctly. Three things before we can merge:

1. The DCO check is failing. Please sign off your commits (git rebase --signoff main, then force-push).
2. Please say in the AI-assistance section which tool you used and for which parts, and add an Assisted-by: trailer (see AI_POLICY.md).
3. Rebase onto current main.

Optional, or as a follow-up: detect('maintainers') still reports confidence 1.0 for this source (context/sieve.py), which #464 asks to lower.

Drafted with Claude Code; reviewed and posted by me.

[akshatshahh]

All three addressed:

1. DCO: rebased onto current main with --signoff and force-pushed; the commit now carries Signed-off-by.
2. AI disclosure: the PR body now names the tool (Muse) and which parts it drafted (the parser fix and the 6 new tests), and the commit has an Assisted-by: trailer per AI_POLICY.md.
3. Rebased onto current main (a5ec003); the 25 sieve tests pass on the rebased branch.

Leaving the confidence-1.0 follow-up for a separate PR as suggested.

[mlieberman85]

Thanks, all three points addressed. The parser fix and tests look good; merging.