Prose style: retire the em-dash habit across comments, docs, and generated text

Owner directive: the " — " construction is overused across this repo's prose. Comments, markdown, commit messages, diagnostics, hover text. The style going forward is to use it very occasionally at most; sentences restructure with parentheses, colons, commas, or a second sentence.

State of the tree: grep -rn " — " docs/ README.md pkg/ cmd/ returns thousands of lines. A single mechanical sweep would churn every file and bury real history, so the retirement is incremental:

1. New text: hold the line in review from now on. Recent PRs (#329, #330) already had their added prose scrubbed pre-merge.
2. User-facing rendered text first: hover/completion docs the LSP renders, CLI help and Long: strings, diagnostics. These are product surface, worth deliberate passes. Note one idiom to decide on rather than sweep blindly: the hover heading separator (**name** · type — description) is a consistent format across the whole hover surface; if it changes, it changes everywhere at once, in one reviewed pass.
3. README and front docs: covered by the README overhaul underway.
4. Package docs and comments: opportunistically, whenever a file is edited for real work; no drive-by churn commits.

The one place a dash still earns its keep: a genuine aside that neither parentheses nor a colon carries. Rare by construction.

Start with the named search across docs/, README.md, pkg/, and cmd/, then inspect the LSP hover and completion text, CLI help and Long: strings, and diagnostics. Done is not a repository-wide sweep: it requires an explicitly scoped, reviewed pass over a user-facing surface while preserving the stated exception for genuine asides.