Add category and per-skill pip extras with manifest sync (#236).

[rosspeili]

Generate optional dependencies from skill manifests via sync_extras.py, add install guide and loader hints, and standardize docs on per-skill install targets.

Description

Closes #236

Implements the category + per-skill optional extras model on one wheel:

• scripts/sync_extras.py — regenerates category, per-skill, and [all] rows in pyproject.toml from manifests (--check for CI)
• skillware/core/extras.py — core-dep filtering, extra naming, loader error hints
• SkillLoader — missing manifest deps suggest pip install "skillware[category_skill]" (+ category / [all] fallbacks)
• docs/usage/install_extras.md — canonical install reference
• Docs ripple: README, CONTRIBUTING, TESTING, examples index, all skill catalog pages (Recommended install uses per-skill extra even when empty today)
• tests/test_extras_sync.py, registry-doc guard for skill-specific install lines

Breaking (clean break): drops legacy cli and embeddings extras; [all] = skill runtime deps only; new meta [agents] for SDK union.

Type of Change

• New Skill
• Skill Upgrade
• Bug Fix
• Documentation
• Framework Feature
• CLI
• Examples
• Packaging
• RFC / meta

Checklist (all PRs)

• Linked GitHub issue (Refs #236)
• Scope matches the issue — no unrelated refactors
• python -m black --check . and flake8 pass locally
• pytest skills/ and pytest tests/ pass locally (274 passed)
• CHANGELOG.md updated under [Unreleased]
• examples/README.md updated (per-skill extras on every script row)
• Ran pytest tests/test_registry_docs.py

New or updated skill

N/A — packaging/framework/docs only.

Constitution and safety (skills only)

N/A

Related Issues

Refs #235, #236