feat: add Continuity Manifest context mode

[dlxeva]

What changed

• Add flg context --mode manifest as a compact project continuity entry point.
• Preserve Source Health, Expand On Demand, and Boundary even under very small budgets.
• Document the Continuity Manifest contract and update the current product/iteration state.

Boundary

The manifest is a derived view. It does not replace the formal ledger or Evidence Index. Existing flg resume behavior remains unchanged.

Validation

• Full test suite: 193 passed
• Smoke test: passed
• git diff --check: passed
• Real project comparison: manifest 5,345 bytes vs resume 12,095 bytes
• --budget 1: safety sections remain present