#23308 cmux NEXT Home: the thought headline is a String extension (fixes the namespace-type base red)

cmux-next checks fails on feat-cmux-next at the package conventions check: namespace-type on CmuxHomeCore/Store/HomeThoughtHeadline.swift:7, an enum with only static members ("extension on the receiver type or an instantiated value"). Every PR carries this red.

• HomeThoughtHeadline.of(_:) and .limit become String.thoughtHeadline and String.thoughtHeadlineLimit in String+ThoughtHeadline.swift, the way String+HomeInternalID.swift reads. The helper lastSentence is a private property.
• Call sites: HomeLiveBook (run.lastThought?.thoughtHeadline), HomeTurnProgress (narration limit and headline(_:)), HomeTurnProgressTests, and a doc reference in HomeLive.

No baseline entry: the baseline says not to add branch debt.

Tests

• scripts/lint-ios-package-conventions.sh on this branch: "OK: no unjustified convention violations" (exit 0).
• The old enum and the new extension compiled side by side agree on 13 sample thoughts (headings, sentences, empty, over-limit, markdown), and the limit stays 72.
• HomeTurnProgressTests and the CmuxHomeCore suites run in CI.

No capture: a rename of a text helper; the same text reaches the screen.

Agent session: cosmic-gray

🤖 Generated with Claude Code

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Summary by cubic

Fixes the namespace-type convention violation flagged by cmux-next checks by converting the all-static HomeThoughtHeadline enum into a String extension.

• HomeThoughtHeadline.of(_:) and .limit become String.thoughtHeadline and String.thoughtHeadlineLimit in String+ThoughtHeadline.swift; the helper lastSentence is now a private property.
• Call sites in HomeLiveBook, HomeTurnProgress, HomeTurnProgressTests, and a doc reference in HomeLive use the extension.
• Behavior is unchanged: the old enum and new extension agree on sample thoughts (headings, sentences, empty, over-limit, markdown), and the limit stays 72.