[DEV-101] Document how to add a new agent harness in the agent-harness package README

[danii1]

Task: DEV-101

Summary: Document how to add a new agent harness in the agent-harness package README

Implementation Details

I have full context now. Writing the README:
These typecheck errors are all pre-existing source files missing Node/Bun ambient types — looks like dependencies aren't installed in this worktree. Installing and re-running:
All green: typecheck ✅, lint ✅ (pre-existing warnings only), 275 tests pass ✅. Let me do a final consistency check against the root README and existing doc conventions:
Done. Created packages/agent-harness/readme.md covering all acceptance criteria:

Contents

• End-to-end checklist: create src/harnesses/<name>.ts, export from harnesses/index.ts, re-export from src/index.ts, registerHarness in registry.ts, plus alias in HARNESS_ALIASES
• Capability flag decision table: supportedModes (fail-closed via assertModeSupported), supportsMaxTurns, constrainedModeAllowsExternalTools (noted accurately that no built-in sets it today), promptFlag, imageInput/buildImageArgs
• Usage-limit detection: extend USAGE_LIMIT_PATTERNS referencing Claude Code session-limit and opencode/AI SDK examples in the module header, including documented gaps (no-timer spend limits, opencode --print-logs hidden errors) and whole-line anchoring to avoid transcript false positives
• Related diagnostics: detect-max-turns.ts, incomplete-implementation (stderr exclusion rationale), open-questions detection; prompt/input conventions (argv over stdin, ENXIO/TUI pitfalls, workingDir vs cwd)
• Copy-from examples: kimi.ts minimal, claude-code.ts full-featured, codex.ts native images; test updates incl. the exact-count gotcha in registry.test.ts
• Edge cases: soft-deprecation alias path (gemini → antigravity precedent), workingDir (opencode --dir, Grok --cwd), sandbox nesting guard
• Verification: bun run test, typecheck, lint

Verification (all pass): typecheck ✅ · lint ✅ (pre-existing warnings only) · 275 tests ✅

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