feat: add W16 artifact evidence foundation

[jmagar]

W16 Cortex Artifact Evidence

Base: 64098b3 (#199)
Validated code head: 6c84827
Current PR head: 1429880 (docs-only final evidence checkpoint on top)

Scope and authority:

• Cortex remains the durable observability/evidence/history plane only.
• No ArtifactCandidate/ArtifactInterchange ownership, registry/publication authority, crawler, or memory/knowledge ownership.
• First slice remains migration-free and reuses canonical logs; migration 49+ remains deferred to measured need.
• Active OTLP PR feat(agent-observatory): ingest OTLP traces and normalize metrics #200/worktree was not touched.

Post-rebase validation:

• Public Identity: PASS.
• Tests: 2980 passed, 0 failed, 2 skipped in 486.103s.
• Doctests: PASS.
• MCP Integration: 131 passed, 0 failed, 4 skipped, 135 total.
• Clippy -D warnings: PASS.
• Formatting + git diff --check: PASS.
• cargo-deny all features: PASS.
• Version sync: PASS, 16 files in sync at 3.13.2.
• Agent-memory symlinks: PASS.
• Rust module-size <= 500: PASS.
• Docs contract: PASS.

Notes:

• Public action-reference drift and private fixture hostnames found by the first post-rebase run were corrected in 6c84827.
• Isolated CARGO_TARGET_DIR was used only to avoid an unrelated sibling-process target lock; no sibling work was changed.
• Fresh GitHub checks on this rewritten head are required before the lane is considered green/mergeable.

[jmagar]

Final W16 validation evidence for the rebased/published head:

• Base: 64098b3fa4471837aec82a6063cc625f02a61f78 (fix: harden actionable error-signature reads #199)
• PR head: 1429880df9cc95bc573888757c0897da364dc66b
• Repository Contract: PASS
• Public Identity: PASS
• Version Sync: PASS
• Formatting: PASS
• Clippy: PASS
• Dependency Check (cargo-deny): PASS
• CodeQL: PASS
• Tests: PASS (5m08s)
• MCP Integration Tests: PASS (8m51s)
• CI Gate: PASS
• No unresolved review threads/reviews
• PR marked ready for review; GitHub reports mergeable_state: clean

The W16 design remains migration-free and continues to reuse canonical Cortex logs. Active OTLP PR #200/worktree was not touched.