v3.4.0

v3.4.0: fix errors, add features, clean dead code

- Fix npm postinstall: Windows asset names (goat-windows-x64.exe, goat-windows-x64-ide.zip) + zip extraction
- Fix doctor.ts: global fetch (no node:fetch), version from constants.ts, MCP config type import
- Fix custom agent loader: project-level priority per docs/AGENTS.md
- Wire custom agent model: override via resolveClientFor hook
- Dead code cleanup: tools.ts dup comments + fdSync, tui.tsx dup ctrl+k, agent.ts delete-loop
- Delete 6 dead modules: ui.ts, collab.ts, mcp-enhanced.ts, session-db.ts, skills.ts, types.ts
- Prune ambient.d.ts: keep only onnxruntime-node/@xenova/transformers
- Fix onboarding.ts: template literals for provider, VERSION constant
- Fix IDE server MIME: use found candidate's extension
- CI: bun test --timeout 30000 to prevent cold flakes
- .gitignore: ignore .bun-build, tmpdir_logs/, goatcode-cli-*.tgz, Ctmp/, brag-output/, tmp_brag_skill/
- Bump version: 3.4.0 in package.json, npm-package/package.json, src/constants.ts
- CHANGELOG.md + README.md updates