fix(scan): use the registered scan ID in agent prompts

[dipeshbabu]

Fixes #290
Fixes #292

Summary

• Build the agent prompt only after the workbench returns the authoritative scan ID.
• Embed that ID directly in standard-scan artifact guidance and deep-scan MCP arguments.
• Keep the corrected prompt when false-positive feedback is appended.
• Cover standard and deep modes with and without feedback.

Root cause

The prompt was created before scan registration and referred to $CODEX_SECURITY_SCAN_ID. That placeholder is not expanded inside structured MCP arguments or agent-authored artifacts, so deep scans could send an invalid literal ID and standard scans could write a different UUID that finalization rejected.

Impact

Deep scans now call start_codex_security_deep_scan with the registered UUID, and all scan modes instruct the agent to use that same UUID in canonical artifacts.

Validation

• pnpm dlx bun test --timeout 30000 ./tests-ts/api.test.ts --test-name-pattern "uses the registered scan ID in standard and deep prompts|encodes valid Unicode Git refs as data before sending the scan prompt|provides only reviewed false positives" — 3 passed
• pnpm exec tsc --noEmit
• pnpm exec prettier --check src/api.ts tests-ts/api.test.ts
• git diff --check

The complete API test file reached 80 passing tests; its remaining nine tests are blocked in this local Windows environment by the existing credential-home ancestor ACL check.

[dipeshbabu]

@mldangelo could you review this pr?