feat(adapters): add Cline CLI adapter (skills-based, no manifest)

[SHcommit]

Problem

ADR Toolkit supports Claude Code, Codex CLI, Gemini CLI, and Antigravity CLI harnesses, but not Cline CLI (cline). A ClinePass subscriber using the CLI can't install the skills/adr-toolkit package through Cline's native mechanism today.

Proposed Solution

Add a adapters/cline/ adapter. Unlike Codex/Gemini/Antigravity (each a manifest + install-time symlink), Cline CLI uses the open "Agent Skills" standard — the same SKILL.md + skills/ convention the toolkit already ships. Verified against the live Cline docs and the installed cline 3.0.61 CLI:

• cline skill forwards to the open skills CLI (npx skills): cline skill add <owner/repo>.
• Skills auto-discover from .cline/skills/ (project) and ~/.cline/data/settings/skills/ (global).
• skills/adr-toolkit/SKILL.md already satisfies Cline (name matches dir, description < 1024).

So the adapter is README-only (like adapters/generic/) — no plugin.json/gemini-extension.json equivalent exists for Cline skills.

Deliverables:

1. adapters/cline/README.md — cline skill add SHcommit/ADR-toolkit (repo public), project symlink into .cline/skills/, and global ~/.cline/data/settings/skills/.
2. tests/unit/test_cline_adapter.py (mirrors test_generic_adapter.py).
3. tests/unit/test_readme.py — Install table lists Cline CLI.
4. README.md — add a Cline CLI row to the Install table.
5. .gitignore — ignore .cline/skills/adr-toolkit install-time symlink.
6. changelog.md / handoff.md updates.

Manual verification: cline 3.0.61 is installed locally — run install → preflight/init/validate in a scratch repo (same depth as the other adapters).

Alternatives Considered

• Cline TS "Plugin" (cline plugin install, package.json cline.plugins) — the AgentPlugin mechanism bundles tools/hooks in TypeScript. Rejected: ADR Toolkit's deterministic core is Python; a TS wrapper is rewrite-for-no-benefit, and the always-on-hook use case is already declined in project-roadmap.md ("Harness parity" → hook support "Evaluated, not pursued").
• Manifest file like the Codex/Gemini/Antigravity adapters — N/A; Cline has no skill manifest format.
• .clinerules/ rule — always-on prose, not on-demand skill discovery; the root AGENTS.md is already auto-detected by Cline.

Additional Context

• I checked project-roadmap.md and improvements.md for existing plans — "Harness parity" tracks Codex/Gemini/Antigravity only; Cline is not covered.
• Cline docs: docs.cline.bot/customization/{skills,plugins,cline-rules}.md, docs.cline.bot/sdk/{plugins,plugin-install}.md.
• Repo is public (SHcommit/ADR-toolkit), so cline skill add SHcommit/ADR-toolkit is viable.
• CI harness-parity extension for Cline is deferred to a follow-up (non-interactive npx skills verification needs design).

[SHcommit]

Implemented in PR #20 (feat(adapters): add Cline CLI adapter), merged into develop. Cline CLI installs skills via the open Agent Skills standard, so the adapter is README-only (no manifest). Manually verified against Cline CLI 3.0.61.