fix(#5202): add platform detection to gitleaks install in post-scripts

[fullsend-ai-coder]

Port platform detection logic from scan-secrets into post-code.sh and post-fix.sh. The gitleaks download previously hardcoded linux_x64, which fails on macOS with exit status 126 (cannot execute ELF binary).

The install block now uses uname -s and uname -m to detect the OS and architecture, selects the correct per-platform checksum from a declare -A associative array, and falls back to shasum on macOS where sha256sum is not available.

Add platform detection unit tests to post-code-test.sh covering all supported platforms, unsupported architectures, and correct checksum selection.

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Closes #5202

Post-script verification

• Branch is not main/master (agent/5202-fix-gitleaks-platform-detection)
• Secret scan passed (gitleaks — 83397e5355462dea996845ed2a1ec019d32f30ac..HEAD)
• Pre-commit hooks passed (authoritative run on runner)
• Tests ran inside sandbox

[github-actions]

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[github-actions]

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[codecov]

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[fullsend-ai-retro]

🤖 Finished Retro · ✅ Success · Started 5:29 PM UTC · Completed 5:39 PM UTC
Commit: 11bf88d · View workflow run →

[fullsend-ai-retro]

PR #5211 was a well-implemented fix for gitleaks platform detection (issue #5202) that was closed without merging because it modified scaffold template copies in internal/scaffold/fullsend-repo/scripts/ instead of the active runtime scripts in fullsend-ai/agents/scripts/. The human reviewer (waynesun09) closed the PR and re-filed the fix as agents repo issue #223. This is at least the 5th occurrence of this pattern (prior: PRs #2417, #2656, #2927, #2979, #3156). The code quality was high — the agent correctly ported platform detection with per-platform checksums, shasum fallback, and tests — but the entire code agent run was wasted because AGENTS.md does not document that post-scripts have migrated to the agents repo. Existing issue #3157 tracks adding this documentation but has not been implemented and its proposed text omits scripts/.

Proposals filed

• Expand #3157 to include scripts/ in the list of migrated scaffold paths (in fullsend-ai/fullsend)
• Prioritize #3157 implementation — 5th wasted scaffold-only PR from the same root cause (in fullsend-ai/fullsend)
• Triage agent should detect scaffold paths in issue bodies and warn before applying ready-to-code (in fullsend-ai/agents)