chore: release v0.2.0 (Gate 6 data-binding) + version-check workflow

[opensesh]

Bumps plugin.json to 0.2.0 so /plugin marketplace update can detect the Gate 6 data-binding release reliably. Without this bump, consumers like BOS have to uninstall+reinstall to pick up content changes — observed mid-flight while wiring BOS-3.0 to the marketplace.

Manifest:

• .claude-plugin/plugin.json: version 0.1.0 -> 0.2.0; description "ten explicit gates" -> "eleven explicit gates"
• .claude-plugin/marketplace.json: description string parallel fix

Changelog:

• CHANGELOG.md: prepend v0.2.0 block documenting the added Gate 6, the design-to-code-data-binder subagent, the new 6-data-binding gate-failure template, the gate renumbering (slice plan -> 7, pre-slice -> 8, pre-swap -> 9, pre-retro -> 10), and the 9-pre-retro.md -> 10-pre-retro.md rename. Breaking-change note for consumers with frozen status.json snapshots.

CI guardrail:

• .github/workflows/version-check.yml: new workflow that runs on PRs to main when plugin source (.claude-plugin/plugin.json, agents/, commands/, skills/, templates/, scripts/, DESIGN_TO_CODE_RULES.md) changes. Fails the PR if plugin.json's version matches main's version. Prevents the v0.2.0 issue from recurring.

Tests: 80 passing, unchanged.