feat(integrity): human-readable colored audit output on a terminal

[jig]

Summary

The integrity audit was a single JSON line to stderr — fine for servers, noisy for interactive use. Now the output adapts to where stderr goes:

• Interactive terminal → a human-readable block via the term package, one field per line, green when integrity is satisfied, red when not:

  ✓ integrity verified
      ref     v1.4.2
      commit  9fceb02da8f3c1…
      signer  alice
  

  ✗ integrity check failed
      reason  … differs from its committed version at "v1"
  

• Redirected (pipe/file) → the structured JSON line is kept unchanged, so log ingestion is unaffected. Matches the "JSON for servers, readable for clients" split.

Honors NO_COLOR / CLICOLOR_FORCE. A terminal failure prints the red block and exits non-zero without the old second Error: line.

Implementation

• lib/term: exported StderrStyle (Go-facing colorizer — stderr TTY + NO_COLOR/CLICOLOR_FORCE aware) and StderrIsTerminal; colorDecision refactored to take an fd (separate stdout/stderr caches).
• command: reportIntegrity/integrityBlock; ErrIntegrityReported sentinel so main suppresses the duplicate error line.

Test plan

• TestStderrStyle / TestStderrStylePlain (term): SGR codes under CLICOLOR_FORCE, plain under NO_COLOR.
• TestIntegrityBlockFormat (command): green success block (header + 3 field lines), no signer line when absent, red failure block with the integrity: prefix stripped.
• Full suite green with and without -tags debugger; verified end to end under a pty (colored block) and a pipe (JSON).

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