Subagents, slash commands, skill scaffolder, and discoverability pass (#28)

Growth-focused additions drawn from studying alirezarezvani/claude-skills —
broaden content types beyond skills, lower contribution friction, and improve
discoverability.

Breadth (content types):
- agents/ — 4 Claude Code subagents (pm-partner, sprint-master, cs-guardian,
  launch-captain) that delegate to the strongest skills and run their helper
  scripts to compute results.
- commands/ — 6 slash commands (/prd, /rice, /sprint-plan, /health-scorecard,
  /retro, /exec-summary).
- install.sh --agent claude now installs skills + agents + commands into ~/.claude/.

Contribution UX:
- scripts/new-skill.mjs (npm run new-skill) scaffolds a SKILL.md that already
  passes SkillCheck.
- package.json exposes npm run entry points (new-skill, skillcheck, build:exports,
  build:web, check).

Discoverability:
- Keyword-rich README H1 (Agent Skills for Claude, ChatGPT, Gemini, Cursor,
  Codex & Hermes), subagent/command count badges, a Subagents & Slash Commands
  section, and a Star History chart. Contributing now points at the scaffolder.

CHANGELOG updated. SkillCheck, exports, and web index all verified in sync.


Claude-Session: https://claude.ai/code/session_016JWn5jRD5tcEFKrubjQ6Px

Co-authored-by: Claude <noreply@anthropic.com>

commands/exec-summary.md

@@ -0,0 +1,8 @@
+---
+description: Compress a document or update into a crisp executive summary.
+argument-hint: [text, decision, or document to summarise]
+---
+
+Apply the **executive-summary** skill to: $ARGUMENTS
+
+Lead with the decision or "so what", then the key points and the ask. Keep it scannable, quantify where possible, and surface risks and the recommendation up front. No filler.