Expose consent-gated Lumin signing workflow

[silverstein]

Summary

• expose six public MCP tools for Lumin authorization, request preparation, one-shot request creation, status polling, and artifact download
• require a fresh exact confirmation before a PDF or signer details leave the device
• keep OAuth access tokens and signed artifact URLs out of tool results, logs, and durable state
• preserve zero automatic create retry, no-overwrite downloads, source/share parity, schemas, manifests, annotations, and user-facing documentation

Safety boundaries

• no live provider call or real signature was performed by this change
• confirmation proves the exact statement and timestamp supplied through the host, not independent human identity
• public PKCE clients use polling; app webhooks remain a future server-only path
• this PR does not publish a release

Verification

• exact final head: afd5474
• focused and adjoining Mac suites: 314/314 passing
• release-gate regressions: 106/106 passing
• full serial Mac Vitest: 181 files and 2,951 tests passed, with 107 intentional skips
• Mac native partition: 62 passed, with 9 intentional skips
• two deterministic MCPB production builds matched at SHA-256 48f7c1c17d0972766c98facdca3a661fbd1506c7c5dc9499a4bef44dbe5bfc00
• exact packaged MCPB smoke passed on macOS with 57 tools and 14 prompts
• GitHub Actions passed the complete gate on Node 20.19 and Node 22.12
• independent exact-head review returned GO with no P0, P1, or P2 findings

Tracks pdf-toolkit-mcp-ik9e. Follow-up pdf-toolkit-mcp-tnpc tracks bounded status-observation retention.