[Extension]: Update linear to v0.4.0

[ashbrener]

Update of the existing community-catalog linear entry from 0.3.0 → 0.4.0. Filing via this template per @mnriem's guidance on #2893. Only version, download_url, and changelog change; id stays linear.

Extension ID: linear
Extension Name: Linear Integration
Version: 0.4.0
Description: Mirror spec-kit feature directories into Linear (filesystem → Linear, reconcile-based, unidirectional).
Author: Ash Brener
Repository URL: https://github.com/ashbrener/spec-kit-linear-sync
Download URL: https://github.com/ashbrener/spec-kit-linear-sync/archive/refs/tags/v0.4.0.zip
License: MIT
Homepage: https://github.com/ashbrener/spec-kit-linear-sync
Documentation URL: https://github.com/ashbrener/spec-kit-linear-sync/blob/main/README.md
Changelog URL: https://github.com/ashbrener/spec-kit-linear-sync/blob/main/CHANGELOG.md
Required Spec Kit Version: >=0.1.0

---

What's new in 0.4.0 (see the release):

• Operator-configurable spec→Linear artifact mapping (resolves the long-standing "spec as Project or Issue?" question), with the default mapping unchanged and frozen.
• A lifecycle fix so Linear issues no longer stick at "in-progress" when a spec's branch differs from its directory name.

extension.yml manifest is valid; a GitHub release with the v0.4.0 tag exists (download archive returns 200). Verified specify-installable from the release URL.