docs(community): Update product-forge to v1.6.0 in community catalog

[VaiYav]

Description

Updates the product-forge community extension entry to v1.6.0 (codename "Bulbasaur"), mirroring the previous catalog bumps (#2012, #2352).

• extensions/catalog.community.json — version 1.5.1 → 1.6.0; download_url → the v1.6.0 tag; provides.commands 29 → 31 (two new commands: design-system-harvest, spec-merge); refreshed description; bumped updated_at.
• docs/community/extensions.md — refreshed the catalog row description to match.

v1.6.0 is a minor, fully additive release (no breaking changes): configurable documentation storage strategies (flat/domain-nested/ddd/workspace) behind a single path-resolution contract, a layered global config, the SDD-flow wave (living spec + traceability, structured journeys → Playwright E2E, FE↔BE contract-first APIs, telemetry MCP wiring, risk-scored gate review, first-class express track), plus a repo-wide consistency pass.

• Release: https://github.com/VaiYav/speckit-product-forge/releases/tag/v1.6.0
• Changelog: https://github.com/VaiYav/speckit-product-forge/blob/main/CHANGELOG.md

Testing

• download_url resolves (HTTP 200) — the v1.6.0 tag and source zip exist.
• extensions/catalog.community.json validated as well-formed JSON after the edit.
• Tested locally with uv run specify --help — N/A, catalog/docs-only data change (no code touched).
• Ran existing tests with uv sync && uv run pytest — N/A, catalog/docs-only data change.

AI Disclosure

• I did use AI assistance (describe below)

The catalog/docs version bump and this PR description were prepared with AI assistance (Claude Code) and reviewed by the extension author (VaiYav). This is a data-only update to the community catalog entry for an extension the author maintains.