Allow negative stability scores instead of clamping at zero

[exisz]

Context\nRecent OpenClaw releases are all showing 0 because evidence penalties exceed the 0-100 score floor, hiding relative differences between bad releases.\n\n## DoD\n- Stability formula no longer floors scores at 0; scores may be negative.\n- Upper cap at 100 remains.\n- UI color/trend rendering handles negative scores without breaking.\n- Typecheck/build passes.\n\nRequested by Exis in #isitstable-com on 2026-06-13.

[exisz]

Implemented on dev in commit 7d15d96.\n\nChanges:\n- Removed the 0 floor from stability score calculation; score is now capped only at 100.\n- Updated score badge tooltip to reflect no lower floor.\n- Made score trend scale dynamically so negative scores stay visible instead of falling out of the chart.\n\nVerification:\n- pnpm exec tsc --noEmit\n- pnpm lint (passes with existing img warning)\n- pnpm build\n\nRefs #138