Backfill bundle tests for six registry skills missing test_skill.py.

[rosspeili]

Adds test_skill.py to the six registry skills that were missing bundle tests: mica_module, pii_masker, synthetic_generator, wallet_screening, pdf_form_filler, and prompt_rewriter.

Each bundle test covers manifest consistency (or manifest schema where skill.manifest is empty), input validation, and offline/mocked execute() paths. Also fixes prompt_rewriter/__init__.py so pytest can collect the bundle.

Fixes #158

Description

Backfills co-located bundle tests so every registry skill ships test_skill.py. No skill logic changes except the prompt_rewriter export fix.

Type of Change (Matches Issue Templates)

• Skill Proposal: New Skill (Contains manifest.yaml, skill.py, and instructions.md)
• Bug Report Fix: Non-breaking change which fixes an execution error or framework bug
• Doc Fix: Documentation Update
• Framework Feature / RFC Updates: Core Framework Update (Changes to base_skill.py, loader.py, etc.)

Checklist (all PRs)

• My code follows the Agent Code of Conduct.
• I have run python -m flake8 . and pytest tests/ locally (or the subset relevant to this change).
• CHANGELOG.md updated under [Unreleased] if this PR changes user-visible behavior.
• examples/README.md is updated if this PR adds, renames, or removes a runnable script under examples/.

New or updated skill (complete only if this PR adds or changes a skill under skills/)

Bundle & metadata

• Skill lives at skills/<category>/<skill_name>/ (copied from templates/python_skill/ or equivalent).
• manifest.yaml has name, version, description, valid parameters, and constitution.
• manifest.yaml includes issuer with real name and email (not template placeholders).
• Optional: short_description in manifest (~80 chars) for skillware list.
• Optional: issuer.github and issuer.org set when applicable.
• requirements and env_vars are documented when the skill needs them.

Logic, cognition, and UI

• skill.py is deterministic Python (no arbitrary LLM-generated code paths).
• instructions.md explains when and how to use the skill.
• card.json is present and its issuer matches manifest.yaml (name and email at minimum).

Tests & loader

• test_skill.py covers execution and schema expectations.
• SkillLoader.load_skill("<category>/<skill_name>") succeeds (or missing deps are documented).

Documentation & catalog

• docs/skills/<skill_name>.md exists or is updated (ID, Issuer, usage).
• docs/skills/README.md lists the skill with ID and Issuer.

Constitution & Safety (if adding or modifying a skill)

N/A — tests only. No skill execution behavior changed.

Related Issues

Closes #158