chore(backmerge): sync develop with master after v1.1.0 release

[SHcommit]

Summary\n\nBack-merge from master to develop after the v1.1.0 release (PR #39 + tag v1.1.0), per AGENTS.md Git Flow ("Release branches merge into master and back into develop"). Brings develop up to master HEAD 71afafc (Merge PR #39).\n\n## What is in this back-merge\n\nThis is a no-ff merge of origin/master into a branch off develop:\n\n- 71afafc Merge pull request #39 from SHcommit/release/v1.1.0\n- 55650d7 chore(release): bump VERSION to 1.1.0 and sync manifests for v1.1.0 release\n\nFile diff vs develop (7 files, +91/-50):\n\n- skills/adr-toolkit/VERSION → 1.1.0\n- skills/adr-toolkit/SKILL.md frontmatter version → 1.1.0\n- .claude-plugin/plugin.json version → 1.1.0\n- adapters/gemini-cli/gemini-extension.json version → 1.1.0\n- adapters/antigravity/plugin.json version → 1.1.0\n- changelog.md new ## v1.1.0 (2026-09-06) section\n- handoff.md updated for the release flow\n\n## Release state\n\n- Tag v1.1.0 is on master at 71afafc (https://github.com/SHcommit/ADR-toolkit/releases/tag/v1.1.0).\n- The release.yml workflow ran successfully and published a GitHub Release with the skill tarball adr-toolkit-skill-v1.1.0.tar.gz + sha256 and Python wheel/sdist.\n- Known issue (follow-up): the Python wheel/sdist are named adr_toolkit-1.0.1-* because pyproject.toml version field is not in scripts/sync_version.py MANIFEST_SPECS scope. The skill tarball version is correct (1.1.0). A follow-up release (v1.1.1 hotfix) should add pyproject.toml to the sync scope and correct the wheel/sdist version.\n- Known issue (follow-up): claude plugin marketplace add /path/to/ADR-toolkit fails on Claude Code v2.1.263 because .claude-plugin/marketplace.json is missing the required owner field (and likely $schema + top-level description) that Anthropic-Code marketplace schema requires (compare Agent-toolkit/.claude-plugin/marketplace.json). This blocks installing the Claude Code plugin locally until fixed.\n\n## Why a PR (not direct push)\n\ndevelop and master are protected by repository rulesets — direct pushes are declined (already encountered when trying to push the release branch). A short-lived chore/* branch is the allowed path. Delete after merge per AGENTS.md.\n\n## Verification\n\n- git status clean before tagging.\n- git merge --no-ff origin/master produced commit 447ffd8d8d57f0b1d77dfced97f1320e23bd2b77 on chore/backmerge-v1.1.0.\n- No code changes beyond the version metadata already on master.\n\n## Examples Impact\n\n- [x] N/A — back-merge PR (chore(backmerge):), no examples/ change.\n\n## ADR Impact\n\n- [x] N/A — back-merge of an already-released release, no new architectural decision.\n