feat: add Cline harness support

[slowdini]

What

Adds Cline as a fourth supported harness, and sets up Cline's own tooling (rules + Memory Bank) for working on this repo with Cline.

Cline plugin

• cline/plugins/slow-powers.js — dependency-free JS AgentPlugin, declared via the cline field in package.json (which is already version-locked). Install with cline plugin install https://github.com/slowdini/slow-powers.git.
• Bootstrap injection: setup() registers bootstrap.md as a session rule via registerRule, so the skill-enforcement block is in every session's system prompt — the Cline equivalent of the Claude/Codex SessionStart hook and the OpenCode system-prompt transform.
• Plan gate: hooks.beforeTool skips the first switch_to_act_mode of each conversation with a reason pointing at hardening-plans, keyed by a per-conversation tmp marker — the same deny-once, fail-open, can't-hard-lock-plan-mode argument as hooks/exit-plan-mode. The skip + reason semantics match the runtime's own core.plan-mode-command-guard extension.
• Skills: auto-discovered from the package root by Cline's plugin loader — no wiring needed.

Repo-local Cline setup

• .clinerules/memory-bank.md — Cline's canonical Memory Bank instructions.
• memory-bank/ — the six core files, seeded; activeContext.md records this work and the verification results.
• AGENTS.md — four-harness update + live-testing instructions. (Cline reads AGENTS.md natively — no symlink needed.)
• .gitignore — excludes .cline/plugins/ install artifacts.

Tests

• tests/harness/spec.ts gains the Cline harness entry; manifests.test.ts asserts cline.plugins[] paths resolve inside the repo with hooks+rules capabilities.
• New tests/harness/cline-plugin.test.ts: plugin shape, bootstrap registration, gate behavior (skip-once, per-conversation independence, missing-id fallback, fail-open on malformed contexts).

Harness tested

Cline CLI 3.0.51 (per PR requirements). Live verification on headless sessions:

• cline plugin install <repo> --cwd <scratch> installs and registers the plugin; --force reinstall picks up edits.
• All 8 skills are discovered and listed in-session; the <EXTREMELY-IMPORTANT> bootstrap block is present in the agent's instructions, and the bootstrap was behaviorally active (agent invoked a skill on a ~1% match, per the bootstrap rule).
• The tool pipeline runs cleanly with the plugin's hook attached; plugin uninstalled and scratch dir removed afterward.

One check remains manual: switch_to_act_mode is not exposed in headless one-shot sessions, so the gate firing end-to-end needs one interactive cline -i plan-mode run (present plan → approve → first mode-switch is skipped with the hardening instruction). The gate logic itself is unit-tested against the documented AgentBeforeToolResult contract, and the runtime's skip handling + hook context shape were confirmed against the shipped CLI source.

Evals

Not applicable — no skill content changed (no behavior-shaping prose touched); this PR adds harness infrastructure only.

Notes for reviewers

• Cline plugins load only in the CLI, SDK, and Kanban — not the VSCode/JetBrains extensions. The README documents the IDE fallback (manual skills copy into .cline/skills/ or ~/.cline/skills/; no bootstrap/gate there).
• Known follow-ups (recorded in memory-bank/progress.md): the already-hardened short-circuit (upstream Claude hook causes issues if hardening-plans is invoked autonomously #153 analogue) for the Cline gate, and a cross-harness exploration of installed-plugin-vs-repo skill precedence (Cline's skill registry is last-wins with plugin dirs scanned after workspace dirs).