chore(skills): Add linear-project-update skill (#21233)

## Summary

Adds a new agent skill, `linear-project-update`, that helps draft and
(with explicit approval) post status updates to Linear projects.

Workflow:

1. Resolve the project from a URL/slug/UUID, or — if nothing is passed —
list projects led by the current user, sorted with In Progress first and
annotated with target date + days-since-last-update.
2. Invoke the existing `linear-project-status` skill on the resolved
project to produce the health verdict and concrete findings.
3. Ask the user once for context Linear doesn't have (PTO, blockers,
competing priorities, etc.).
4. Draft a target-date recommendation (defaulting to "no change";
cautioning against sub-week moves) and an update body in the lead's
voice.
5. Present both proposals and offer Submit / Adjust / Stop. Apply
changes only on explicit go-ahead in the same turn.

The user keeps full ownership of the words — the skill drafts a proposal
and never posts without confirmation.

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Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
Co-authored-by: Sigrid <32902192+s1gr1d@users.noreply.github.com>