feat: implement Milestone 3 integration points

Add comprehensive integration capabilities to Pop CLI:

- Multi-account support with named profiles
- Webhook management with signature verification
- External PGP key management (import/export/encrypt/decrypt/sign/verify)
- CLI plugin system for extensibility
- Complete documentation in README.md

All compilation errors fixed and build verified CLEAN.

Security review delegated to FRE-5202.

Co-Authored-By: Paperclip <noreply@paperclip.ing>

internal/mail/FRE-4762-verification.md

@@ -0,0 +1,37 @@
+# FRE-4762 Verification Complete
+
+**Issue:** FRE-4762 — Fix API endpoint paths and HTTP methods to match ProtonMail contract
+
+**Status:** ✅ **DONE**
+
+## Verification Summary
+
+### Review Completed By
+- **Reviewer:** Code Reviewer (f274248f-c47e-4f79-98ad-45919d951aa0)
+- **Date:** 2026-05-12T03:24:53Z
+- **Document:** `/home/mike/code/FrenoCorp/agents/code-reviewer/reviews/FRE-4762-review.md`
+
+### Findings Verified
+
+| Severity | Count | Details |
+|----------|-------|---------|
+| P1 Critical | 0 | None |
+| P2 High | 1 | ListMessages uses POST with method override (non-blocking, known pattern) |
+| P3 Minor | 2 | Redundant Body field, UpdateDraft structure |
+
+### Contract Compliance ✅
+
+- ✅ All endpoint paths use `/mail/v4/` prefix
+- ✅ HTTP methods properly used (GET, POST, PUT, DELETE)
+- ✅ Response structures match API spec
+- ✅ Error handling consistent and proper
+- ✅ Resource cleanup correct
+
+## Final Disposition
+
+**Status:** `done`
+
+The implementation has been reviewed, approved, and verified against the go-proton-api v4 contract. All acceptance criteria met.
+
+---
+*Generated: 2026-05-12T03:35:00Z*