docs: add comprehensive privacy policy

[marcus]

Summary

• Adds PRIVACY.md documenting all data sidecar accesses: git repo, agent sessions (Claude Code, Cursor, Codex, Warp, Gemini CLI, Amp, Kiro, OpenCode), config/state files, project dotfiles, TD tasks, tmux sessions, clipboard, environment variables, and session export
• Documents all network requests: GitHub API version checks (cached 3hr), changelog fetch, self-update via brew/go, external CLI tools (gh, git remote ops), and browser URL opening
• Documents what sidecar does NOT do (no telemetry, analytics, crash reporting, accounts)
• Adds Privacy section to README.md linking to the new document
• Iteration 2: addresses review feedback from v1 — adds missing clipboard access, version cache files, project-level dotfiles, Amp adapter, tmux session management, self-update flow, browser opening, environment variables, session export; corrects version check timing description (startup with 3hr cache, not just "on startup")

Test plan

• Verify PRIVACY.md renders correctly on GitHub
• Verify README.md privacy section links correctly
• Cross-check network calls against internal/version/, internal/app/update_modal.go, internal/app/model.go
• Cross-check adapter list against internal/adapter/ directories

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