Add mail CLI and config for gmail_handler operator UX (#292)

[rosspeili]

Description

Refs #292

Adds skillware mail for office/gmail_handler: init and manage address book and email signatures from the CLI, without editing files inside the installed package.

Operators can create persistent config under their user directory (~/.config/skillware/ or %APPDATA%/skillware/), add contacts, and set plain + HTML signatures (Skillware logo, tagline, disclaimer, links). skillware config show displays merged mail.* settings. The skill reads those paths and appends signatures on new outbound mail only.

Gmail’s web signature does not apply to SMTP sends; this gives agents a dedicated, skillware-managed signature instead.

Type of Change

• New Skill — new registry bundle under skills/
• Skill Upgrade — changes to an existing skill under skills/
• Bug Fix — incorrect runtime or framework behavior
• Documentation — docs, README, CONTRIBUTING only
• Framework Feature — skillware/core/ loader, env, adapters
• CLI — skillware/cli.py, docs/usage/cli.md
• Examples — examples/*.py, agent loops, examples/README.md
• Packaging — PyPI wheel, pyproject.toml, MANIFEST.in
• RFC / meta — templates, labels, CI, or large design doc

Checklist (all PRs)

• Linked GitHub issue (Fixes #… or Refs #…)
• Scope matches the issue — no unrelated refactors
• python -m black --check . and flake8 pass locally (or CI-equivalent subset)
• pytest skills/ and pytest tests/ pass locally when relevant
• CHANGELOG.md updated under [Unreleased] when user-visible behavior changes
• examples/README.md updated if this PR adds, renames, or removes a runnable script
• Ran pytest tests/test_registry_docs.py when skills, examples index, or agent-loops matrix changed

New or updated skill

Bundle and metadata

• Skill at skills/<category>/<skill_name>/ (from templates/python_skill/ or equivalent)
• manifest.yaml: name (full ID), version, description, parameters, constitution, real issuer
• Optional: short_description, issuer.github, issuer.org, requirements, env_vars

Logic, cognition, tests

• Deterministic skill.py (no ad-hoc LLM-generated execution paths)
• instructions.md explains when and how to use the skill
• card.json issuer matches manifest when present
• test_skill.py covers execution and schema expectations
• SkillLoader.load_skill("<category>/<skill_name>") succeeds (or deps documented)

Documentation and catalog

• docs/skills/<skill_name>.md and row in docs/skills/README.md
• Usage Examples for Gemini, Claude, OpenAI, DeepSeek, Ollama per skill usage template

Constitution and safety (skills only)

Dedicated agent mailbox only; preview/confirm before send; recipient cap unchanged. Signatures apply to new mail only, not replies (follow-up issue).

Related Issues

Closes #292