(harnesses): CLI harness port and Claude adapter, no behaviour change #820

First step of the Codex CLI support port (upstream ac72985, f5e38a7; credit Ali Basiri). No visible change.

The fork diverged too far to cherry-pick: upstream moved read-session-file.js/derive-project-path.js into harnesses/claude.js; the fork extended both. Re-implement as adapters around fork code:

• harnesses/{index,claude}.js with upstream's names, plus harnesses/port.js (JSDoc typedefs: TranscriptStore, TranscriptParser, Launcher, ActivityClassifier required; UsageProvider optional; capabilities flags such as subagents, continuations, liveElsewhere, bgJobs, ideBridge, sandbox, schedule, remote) and test/harness-contract.test.js run against every adapter. UI and main gate on capabilities, never on runtime === 'claude'. Adapters stay pure (no Electron, no DB).
• Extract a pure Claude argv builder from main.js open-terminal; the fork's worktree/sandbox/pre-launch/MCP wrapping stays outside the adapter. Characterization tests: same argv as today for fork, resume, start-fresh, worktree-not-git, add-dirs.
• runtime TEXT DEFAULT 'claude' and sessionFile TEXT columns via the writer's column reconcile (ADR 0004: no mustReindex, schema only through the writer); an old schema gains both with no row loss.
• harnessForFolder composes with parseFolderKey (alias::x).
• package.json build.files includes harnesses/**.