test: migrate Python golden harness to tryscript and harden port-prep specs

[jlevy]

Summary

This PR prepares repren as a high-fidelity source-of-truth for the Rust port by migrating the Python golden harness to tryscript and hardening the port-prep documentation/spec workflow.

Key Changes

• Migrated golden integration tests from bash to tryscript:
  • removed legacy harness files: tests/run.sh, tests/golden-tests.sh, tests/golden-tests-expected.log
  • added modular suites under tests/tryscript/:
    • help-errors.tryscript.md
    • replacements.tryscript.md
    • renames-and-full.tryscript.md
    • patterns-and-case.tryscript.md
    • walk-and-filters.tryscript.md
    • backups-undo-clean.tryscript.md
    • json-output.tryscript.md
    • regex-wordbreaks.tryscript.md
• Migrated fixtures from tests/work-dir to tests/tryscript/fixtures.
• Added deterministic tryscript config at repo root:
  • tryscript.config.js
• Added golden quality gate script:
  • scripts/check-golden-coverage.sh
  • enforces required module presence, key CLI flag coverage, and no bare ... elisions.
• Updated project wiring:
  • Makefile: test-golden, test-golden-coverage, update-golden; make test now runs pytest + golden gate + tryscript
  • CI (.github/workflows/ci.yml): runs pytest, golden coverage gate, and tryscript
  • tests/pytests.py: integration test now runs tryscript suite
• Added/updated planning artifacts for Rust parity prep:
  • new active migration spec for tryscript
  • rust-port prep spec/research updates and supersession annotations
  • synced/closed the associated tbd issue tree

Why

• Golden sessions are now easier to review and maintain.
• Behavior domains are explicit, making Python→Rust parity mapping straightforward.
• Golden discipline is enforced with automated coverage/anti-pattern checks.

Validation

• uv run python devtools/lint.py
• uv run pytest
• bash scripts/check-golden-coverage.sh
• npx tryscript@latest run tests/tryscript/*.tryscript.md

All checks pass locally.