[Extension]: Update linear to v0.5.0

[ashbrener]

Update of the existing community-catalog linear entry from 0.4.0 → 0.5.0. Follows #2931 (the v0.4.0 bump, now merged). Only version, download_url, and changelog change; id stays linear.

Extension ID: linear
Extension Name: Linear Integration
Version: 0.5.0
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/v0.5.0.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

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What's new in 0.5.0 (see the release):

• Author-based attribution (spec 010) — opt-in author:<handle> label + author-as-assignee-on-create so the board reflects who authored each spec (default OFF; existing installs unchanged).
• ADR / decision-record mirroring (spec 008) — each spec's research.md decisions mirror as ADR comments on its Linear Issue.
• Consumer identity-leak hardening (feat: Add comprehensive template validation system #69) — an install-time guard keeps operator identity out of committed files.

All additive and opt-in; extension.id stays linear; the command surface is unchanged. extension.yml manifest is valid; a GitHub release with the v0.5.0 tag exists (download archive returns 200). Verified specify-installable from the release URL.