Implement canonical mecatui status surfaces

[jbeda]

Implementation

Implements the approved Plan / Interface PR #2070 from full approved baseline 1caaa60004e36c272a9a05b3f7e28b40b4e1e9c9.

• Removes the duplicate nil-source header and fitFooter generators and their legacy-only formatting helpers/tests. An explicitly source-free UI shows a safe short session handle but no generated right-footer status; normal mecatui uses the stock or configured status source.
• Keeps the debug target warning and privacy disclosure outside customizable templates, budgets the warning before template/command variant selection, and keeps both visible in feasible narrow/fatal frames.
• Corrects unknown/estimated context-meter readings across responsive stock variants. Migrates meaningful source-backed coverage, provides a deterministic stock-status golden fixture, and replaces legacy-only goldens with no-source snapshots where applicable.

ACs completed: AC1.1–AC1.3, AC2.1–AC2.3, AC3.1–AC3.2. The landed candidate's task ac-trace-strict resolves all eight proofs.

Interfaces match approved contract: Yes. No implementation deviation required a plan amendment. No gRPC, exported Go signature, tool schema, CLI/config, persistence, or debugger authority change.

Verification

• task docs — passed
• task site:build — passed (npm reports 47 dependency audit advisories; no dependency changes in this PR)
• task test — passed on final candidate
• task lint — passed on final candidate
• task test:race — passed on final candidate
• go run ./cmd/mecademo — passed; tool call, permission ask/approval, and result observed
• task ac-trace-strict — passed on final candidate
• /panel-review: Spec, Standards, Security, Architecture, Library reuse, and Duplication clear. Test adequacy raised three important findings; two focused repair rounds resolved all three and the reviewer validated the final candidate. PANEL: ship_blockers=0 important=0 advisory=0 reviewer_failures=0

The 105-file diff includes intentional snapshots of source-free test models and deletes four obsolete legacy-footer goldens. It does not change terminal layout outside the approved status presentation contract.