v0.2.5 — CONTRIBUTING.md + actionable marker hints + repo hygiene

[0.2.5] - 2026-05-16

Added

• CONTRIBUTING.md — human-friendly contributor entry point pairing
  with AGENTS.md (the AI-agent rules). 60-second checklist, what we
  welcome / push back on, security-issue reporting.
• timeline marker add error hints are now actionable. When AddMarker
  returns false, the CLI now inspects the timeline state and emits one of
  two specific hints instead of the generic "confirm preconditions" line:
  • timeline has zero clips → "Timeline 'X' has no clips. Resolve markers must anchor to a clip — import or append at least one clip before placing a marker. …"
  • timeline has clips → "AddMarker returned false. Common causes: marker already exists at this frame, frame outside the timeline's clip range, or the color string is misspelled."
• ApiCallFailed now accepts a hint= keyword argument so domain code can
  attach situation-specific guidance without forking the error class.

Removed

• test-cases.md at the repo root. It was internal AC↔pytest mapping
  scaffolding that leaked into the public repo from v0.1; AC are tracked
  in the milestone files under versions/v*/milestones/ and the test
  files themselves.

Tests

• 186 (was 184). Two new tests cover the marker-add hint branches.

No public API behavior changes besides the richer hints — no breaking changes.