Batch A: blob storage layer, MCIAS auth, OCI token endpoint

Phase 2 — internal/storage/:
Content-addressed blob storage with atomic writes via rename.
BlobWriter stages data in uploads dir with running SHA-256 hash,
commits by verifying digest then renaming to layers/sha256/<prefix>/<hex>.
Reader provides Open, Stat, Delete, Exists with digest validation.

Phase 3 — internal/auth/ + internal/server/:
MCIAS client with Login and ValidateToken, 30s SHA-256-keyed cache
with lazy eviction and injectable clock for testing. TLS 1.3 minimum
with optional custom CA cert.
Chi router with RequireAuth middleware (Bearer token extraction,
WWW-Authenticate header, OCI error format), token endpoint (Basic
auth → bearer exchange via MCIAS), and /v2/ version check handler.

52 tests passing (14 storage + 9 auth + 9 server + 20 existing).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

PROJECT_PLAN.md

@@ -11,8 +11,8 @@ design specification.
 |-------|-------------|--------|
 | 0 | Project scaffolding | **Complete** |
 | 1 | Configuration & database | **Complete** |
-| 2 | Blob storage layer | Not started |
-| 3 | MCIAS authentication | Not started |
+| 2 | Blob storage layer | **Complete** |
+| 3 | MCIAS authentication | **Complete** |
 | 4 | Policy engine | Not started |
 | 5 | OCI API — pull path | Not started |
 | 6 | OCI API — push path | Not started |