docs: tighten README — drop defensive framing, lead with concrete claims

[pmclSF]

Summary

Tightening pass on the v1 rewrite (PR #180) after feedback that it was still too defensive and a little vague.

• Opens with three concrete behavior claims (change a prompt, add an agent, touch a training pipeline) instead of an abstract pitch
• Cuts the "doesn't replace Ruff / ESLint" hedge — confident products don't pre-disclaim what they aren't
• Cuts the "What Terrain doesn't do" section entirely (six anti-claims that say nothing positive)
• Trims the detector list from 11 to 5 — only the boundary-gap moat detectors, not the AI hygiene patterns that overlap with Semgrep / secret scanners
• Cuts Snapshots, Policy, Architecture, and What's-stable-in-0.2 sections — all link out to docs

Length: 169 → 85 lines.

Test plan

• Render check on GitHub
• Detector names still match `internal/signals/signal_types.go`
• All linked docs paths exist

🤖 Generated with Claude Code

[github-actions]

[INFO] Terrain — Informational only

Insufficient data to assess change risk confidently.

Metric	Value
Changed files	1 (0 source · 0 test)

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✓ All clear. No new findings introduced; no protection gaps identified in changed code.

Run terrain compare over time to track posture; this clean state is the bar to hold.

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Limitations

• No coverage artifacts provided; protection gaps reflect missing data, not measured absence. Provide --coverage to improve accuracy.
• Mixed test cultures reduce cross-framework optimization confidence. Consider standardizing on fewer frameworks.

_{Generated by Terrain · terrain pr --json for machine-readable output}

Targeted Test Results

No tests selected — change affects only non-code files.

[github-actions]

Terrain AI Risk Review

Metric	Value
AI surfaces	13
Eval scenarios	17
Impacted scenarios	0
Uncovered surfaces	13

Decision: PASS — AI surfaces are covered.