chore(release): v1.2.1

[RichardHightower]

Summary

Prepares the v1.2.1 patch release. Bumps pyproject.toml version 1.2.0 → 1.2.1 and adds the CHANGELOG entry.

1.2.1 is an eval-driven patch release. Four bugs surfaced by dogfooding designdoc against its own repo are fixed; the ULTRAREVIEW follow-up hardening pass also lands.

Eval-driven fixes (user-visible)

• #49 Hermetic mode for non-MCP agents — setting_sources=[] blocks user ~/.claude/CLAUDE.md and output-style leakage into doer/checker subprocesses. Eval: 4 → 0 stray Insight blocks in class docs.
• #46 Stage 3 doer cwd plumbing — SDK subprocess now inherits cwd=target_repo so Read calls resolve when target ≠ working dir.
• #41 Stage 2 tolerant JSON extraction — strips code fences / preambles before pydantic validation. Schema-retry rate ~94% → <5% on real-repo eval.
• #42 DEFAULT_EXCLUDES covers .claude/, .opencode/, .devcontainer/, .idea/, .vscode/.

ULTRAREVIEW hardening (issues #6-#18)

state-save lock, TaskGroup budget cancellation, Stage 7 checkpoint parity, retry-counter split, MermaidIssue typing, guard tests, silent-failure cleanup, helper extractions. Full list in CHANGELOG.

Release plumbing (already merged but enumerated for the changelog)

• publish-on-tag PyPI release workflow (.github/workflows/release.yml)
• plugin marketplace metadata (.claude-plugin/plugin.json, marketplace.json)

Invariants preserved

• MAX_ATTEMPTS = 3 unchanged. Guard test test_config_does_not_expose_max_attempts still passes.
• Doer/checker isolation unchanged.
• Schema-validated verdicts unchanged; tolerant JSON extraction is parser-side, doesn't loosen schema.
• Mermaid two-checker rule unchanged.

Test plan

• task ci green locally (106 passed in 46.90s)
• CI gates pass on PR
• After merge: tag v1.2.1 and push → release.yml builds wheel + sdist and publishes to PyPI via Trusted Publisher OIDC
• Verify pip install designdoc==1.2.1 works post-publish

Notes

PyPI Trusted Publisher must be registered before tag push. CONTRIBUTING.md → "Releasing" walks through it.