Update docs for dual override mechanism (approval + LGTM)

[jdelfino]

Summary

Update all documentation to reflect that both PR approval and LGTM comments serve as human override mechanisms. The design doc, README, and CLAUDE.md all reference "PR approval as universal override" and need to cover both options.

Files to modify

• docs/design.md
• README.md
• CLAUDE.md

Implementation steps

1. docs/design.md:

   • Update "The approval as override" section (lines 351-358) to describe both mechanisms
   • Update the override mechanism description (line 232) to mention LGTM
   • Update human workflow (line 239): "approving PR review" → "approving PR review or commenting LGTM"
   • Update example code block (lines 285-298) to show both checks
   • Update escalation gradient table (lines 362-367): "approve" → "approve or LGTM"
   • Add note about LGTM staleness: comments posted before the head commit's date are stale

2. README.md:

   • Update line 72: mention LGTM alongside approval
   • Update line 82: "PR approval as universal override" → "Human override (PR approval or LGTM comment)"

3. CLAUDE.md:

   • Update line 39: "PR approval as universal override" → "Human override (PR approval or LGTM comment)"

Acceptance criteria

• Design doc explains both override mechanisms and their staleness semantics
• README mentions both options
• CLAUDE.md key design decisions updated
• No remaining references to "approval" as the sole override mechanism in docs

Dependencies

None — docs can be written independently, but should land after or with the code changes.