Add INDEX.md grouping skills by category for discoverability

[mgiovani]

Problem

With 59 skills in a flat skills/ directory, there is no way to quickly answer the questions a new user naturally asks:

• "Which skill creates a PR?"
• "Is there a debugging skill?"
• "What do all these forge-* skills do?"

The README lists categories but only as a summary paragraph — not as a navigable reference. A flat list of 59 skills is overwhelming to mentees encountering cc-arsenal for the first time.

Proposed solution: INDEX.md

Create an INDEX.md at the repository root that groups every skill by category, with one-line descriptions and links to each skill directory.

Suggested categories (based on actual skills in skills/)

Category	Skills
Git & PRs	git-commit, git-create-pr, git-release, git-sync, fixpr, merge-pr
Code Quality	review-code, review-security, review-deps, review-perf, refactor
Testing	test-suite
Implementation	implement-feature, fix-bug
Forge (Spec-Driven)	forge-architect, forge-brief, forge-dev, forge-qa, forge-review, forge-security, forge-story
Documentation	docs-adr, docs-check, docs-diagram, docs-init, docs-rfc, docs-update
Planning	project-planner, forge-story, forge-brief
Infrastructure	ci-generate, db-migrate, docker-init, env-setup, dokploy-deploy
GitHub / Jira	gh-daily, jira-daily, jira-todo, jira-cli, issue, ideas, board
Teams	team-implement, team-review
Status & Budgets	audit, usage, budget-sweep, scheduler, self-improve
Utilities	create-skill, create-command, create-rule, find-skills, agent-browser, nanobanana, md, ask
Orchestration	add-project, review, fixpr, merge-pr

Entry format

Each entry in INDEX.md should be:

| [`skill-name`](skills/skill-name/) | One-line description of what it does |

Recommended starter set callout

Include a highlighted callout box (or > [!TIP] block) identifying 5-8 skills ideal for first-time users. Suggested: implement-feature, fix-bug, git-commit, git-create-pr, review-code, docs-init.

Acceptance criteria

• INDEX.md exists at the repository root
• Every one of the 59 skills in skills/ is listed and categorized
• Each entry includes at minimum a skill name (linked to its directory) and a one-line description
• A "Recommended starter set" section or callout is present for new users
• README.md links prominently to INDEX.md (e.g. "Browse all 59 skills → INDEX.md")