fix: make tirith block verdicts approvable instead of hard-blocking

[teknium1]

Summary

Fixes the Discord-reported issue where curl -fsSL https://mandex.dev/install.sh | sh was hard-blocked with no way to approve. Reported by pistrie.

Before: Tirith exit code 1 (block) → immediate rejection, no prompt, agent tries another approach.
After: Tirith block/warn → approval prompt with full findings, user can approve or deny.

What changed

• tools/approval.py: Removed the hard block path for tirith. Both block and warn verdicts now go through the approval flow. New _format_tirith_description() builds rich descriptions from tirith's JSON findings (severity, title, description, safer alternatives).

• cli.py: Startup now warns when tirith is enabled but not available (⚠ tirith security scanner enabled but not available).

• Tests: Updated test_command_guards.py — old hard-block tests replaced with approval-flow tests, plus new gateway approval_required test for the exact scenario reported.

Live test

The approval prompt now shows:

⚠️  Dangerous Command

Security scan — [MEDIUM] Lookalike TLD detected: Domain uses '.dev' TLD...;
[HIGH] Pipe to interpreter: curl | sh: Command pipes output from 'curl' directly
to interpreter 'sh'. Downloaded content will be executed without inspection.
  Safer: tirith run https://mandex.dev/install.sh;
pipe remote content to shell

❯ Allow once
  Allow for this session
  Deny

Test plan

• 92/92 approval+tirith+yolo tests pass
• Full suite: 6503 passed (29 pre-existing failures from missing optional deps)