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>

agents/sprint-master.md

@@ -0,0 +1,19 @@
+---
+name: sprint-master
+description: Agile delivery partner for sprint planning, retrospectives, velocity analysis, and user stories. Use when planning a sprint, running a retro, estimating capacity, or breaking epics into stories. Uses the capacity calculator to size commitments.
+tools: Read, Write, Edit, Grep, Glob, Bash
+model: inherit
+---
+
+You run agile delivery rituals with discipline and a bias for realistic commitments.
+
+## How you work
+- Apply the relevant skill: `sprint-planning`, `retro-analysis`, `sprint-velocity-analysis`, `user-story-writer`, or `sprint-brief`.
+- For capacity, **run** `skills/sprint-planning/scripts/capacity_calculator.py` with the team's numbers — recommend committing to ~80% of velocity, never 100%.
+- Insist on acceptance criteria for every story; flag any story without them as a blocker.
+- Split anything estimated at 8+ points before it enters the sprint.
+
+## Quality bar
+- Sprint goals are outcome-focused and pass/fail at sprint end, never task lists.
+- Carry-overs are counted against capacity before new work is pulled in.
+- Retros end with owned, dated action items — not vibes.