v0.8: Add observation, citation, and freshness model

[spytensor]

Context

Every console signal must be explainable: where did it come from, when was it observed, and how much authority does it have. This prevents the future console from showing fake confidence.

Acceptance Criteria

• AC-1: Add an observation/citation/freshness model usable by health signals and snapshot rows.
• AC-2: GitHub facts can cite issue, PR, check URL, head SHA, and observed-at timestamp.
• AC-3: Local validation is clearly distinct from GitHub CI and cannot masquerade as remote CI.
• AC-4: Evidence Packet claims can carry commit/source/timestamp or explicitly report missing freshness.
• AC-5: Source drift citations preserve source id, pin, finding kind, and trust level.
• AC-6: Tests cover fresh, stale, missing, and unknown observations.

Evidence Required

• Model/schema changes.
• Tests for citation/freshness cases.
• Validation commands.
• Risk and rollback.

Tracker Update Required

This issue is incomplete until tracker #238 is updated.

• Tick this issue checkbox in v0.8.0 - CoreRoom Console Data Plane Tracker #238.
• Update any satisfied milestone AC in v0.8.0 - CoreRoom Console Data Plane Tracker #238.
• Update the Evidence Ledger row in v0.8.0 - CoreRoom Console Data Plane Tracker #238.
• Mention the tracker update in PR body.

If implementation is complete but tracker is stale, report implementation complete, tracker incomplete.

Out Of Scope

• No network fetching.
• No silent remote source refresh.
• No semantic correctness claim from citations alone.