chore: prepare 0.7.3 patch release

[rlippmann]

What changed

• Prepared patch release metadata for 0.7.3.
• Added shared conformance hardening coverage across:
  • checkpoint JSON wrapper behavior
  • controller API fixtures (step, preview, state_diff)
  • preprocessor parse boundary fixtures
  • public API fixture membership for has_pending_clarification
• Updated AGENTS.md guidance to explicitly require:
  • repository PR template usage
  • repository issue template usage (bug_report / feature_request)
• Applied a documentation language pass across user-facing docs to prefer direct, concrete, active phrasing.

Why

• This release packages completed conformance hardening needed for stronger Python ↔ TypeScript parity on 0.7 surfaces.
• Explicit template guidance removes ambiguity and keeps contribution formatting consistent.
• Documentation wording updates align with the repository documentation style policy.

Checklist

• pre-commit run (uv run pre-commit run --all-files)
• tests pass (uv run pytest)