feat(mcp): add roots-aware workspace detection

[JSONbored]

Parent roadmap: #127
Parent phase: #234

Goal

Use MCP roots where clients expose them so local branch analysis can understand the intended workspace boundary without guessing or leaking paths.

Requirements

• Detect MCP client roots when available.
• Validate that local git/workspace analysis stays inside the selected root.
• Preserve current CLI behavior when roots are unavailable.
• Redact local absolute paths from API payloads, logs, and public output.
• Keep GITTENSORY_UPLOAD_SOURCE=true unsupported and fail-closed.

Acceptance criteria

• Roots-aware clients get safer workspace selection.
• Non-roots clients keep working through existing cwd/config behavior.
• Source contents are never uploaded.
• Local absolute paths are not included in public-safe output or API payloads.

Validation expected

• MCP roots fixtures.
• CLI fallback fixtures.
• Local path redaction tests.
• Source-upload fail-closed tests.
• npm run test:ci.