Skills: full detail page with diff-confirmed saves

[TheGreatAxios]

Linear: https://linear.app/abklabs/issue/CL-6416

/skills/<name> was a placeholder. It is now the real skill page, built on
the versioning that already shipped — a skill's git history is its version
store, so this adds no storage and no version table.

What's here

• Content editor for a skill's description and body. The buffer is
  newline-normalized, so the bytes reviewed in the diff are the bytes saved.
• Diff-confirmed saves. "Save…" in the top bar opens a review step
  showing the diff between the published version and the editor buffer, with
  Confirm & save / Keep editing. The commit happens only on confirm.
• A head precondition on that confirm. The save carries the version it
  was reviewed against; PUT /:name takes an optional expectedHeadSha and
  answers 409 rather than writing when someone else published in between. The
  page then re-reads, keeps the edit, and re-opens the review against what is
  actually published, saying why — a confirmed diff can never bury the
  version it was not shown.
• Version list — note, author, and time per commit — with Restore
  (the existing route) and Compare, which diffs any earlier version
  against the current one through the same renderer.
• GET /:name/versions/:commitSha reads a skill as it stood at one
  commit, writing nothing; the version id is parsed at the route and a commit
  outside the skill's history is a 404.
• One editor, not two. The roster's inline skill panel and the create
  dialog's edit mode are gone; the roster lists, the page edits. Only the
  skill entry leaves detail-placeholders.tsx — the other placeholders stay
  for their own tickets.

@corbits/text-diff

Build vs. buy: the MIT diff package would have been a fine choice and
nothing here is novel — I hand-rolled it because the tree carries no diff
dependency and this needs ~120 lines with no runtime deps, so a workspace
package is cheaper than a new third-party dependency in a public repo; if
this grows word-level or three-way diffing, swapping in diff behind the
same diffText signature is the exit.

It is bounded on purpose: newlines normalized, identical head and tail
trimmed before anything is allocated, then a hard cap (1500 changed lines per
side, 400k characters) that refuses the quadratic walk and returns an honest
"too large to show line by line" summary instead. Long unchanged runs collapse
to one row, the table is Int32Array-backed (~9MB at the capped worst case),
and the script is computed once — rows and the change summary read off the
same result.

Measured after the trim and cap (was 420MB at 6k lines / 1.3GB at 12k):

input	result
6k lines, one line edited	diffed, 2.3ms
12k lines, one line edited	diffed, 1.1ms
20k lines, one line edited	diffed, 1.3ms
6k / 12k lines wholly rewritten	refused in <0.5ms, summary shown
1400 lines rewritten (just under the cap)	diffed, 14ms

Tests

Diff-utility unit tests including the cap and the trim-shaped perf case; the
dialog flow (save opens the review with the diff, cancel keeps the edit,
confirm writes, a lost race is refused and re-diffed against the winner); the
version list from a stubbed registry; restore and compare hitting their
routes; CRLF neither reading as a change nor changing the bytes written; side
actions failing locally without discarding a dirty draft; and the
roster/route suites updated to the roster-only contract.

bun run check green.

Follow-up

DiffView is workbench-local for now; promoting it to @corbits/react-ui
per the DESIGN.md rule is CL-6449
(https://linear.app/abklabs/issue/CL-6449) rather than part of this PR.
The ticket's "pinned consumers behind latest" nudge is also not here — the
pinnedBy payload carries no pinned-version data, so there is nothing honest
to render yet.

https://claude.ai/code/session_01Shhie5zM8L54bLHq5gFQti