chore: bootstrap skills library — 19 skills + installer + CI auto-tag

Phase 1 of mathias/skills extraction (infra#62 Track D — homelab
next-step plan addendum). Imports ~/dev/.skills/ verbatim (19 skill
dirs + SKILLS_INDEX.md) and adds the installation surface:

- Taskfile.yml — install / update / list / release / check targets
- install.sh — bootstrap installer for hosts without Task. Idempotent
  symlink wirer; default checkout at ~/.local/share/skills/ on every
  host; SKILLS_REF env var pins a tag (default: main).
- .gitea/workflows/release.yml — auto-tag every push to main by
  Bump-Type footer (major/minor/patch, default patch). Skipped when
  commit contains [skip-release].
- README — usage, versioning, contribution flow, secret-hygiene rule.

Phase 1 wires Claude Code only (~/.claude/skills/<name> global +
<repo>/.claude/skills/<name> per-repo). Phase 2 adds Crush, opencode,
antigravity, and gitea-resident agents (cobalt-dingo, agentsquad)
once their skill conventions are researched.

Public repo, markdown-only — no secrets, no client names. Verified
via pre-push grep before initial push.

[skip-release]

SKILLS_INDEX.md

@@ -0,0 +1,91 @@
+# Engineering Skills Index
+
+This index lists all available engineering skills. Load the full SKILL.md on demand.
+
+| Skill | Description | Use when |
+|-------|-------------|----------|
+| `tdd` | Test-Driven Development enforcement | Starting any new feature or fixing a bug |
+| `clean-code` | Clean code principles with Go adaptation | Code review, refactoring, new code |
+| `solid` | SOLID principles with Go examples | Design review, architecture decisions |
+| `code-review` | Code smell detection + Go-specific checklist | Pre-merge review |
+| `refactoring` | Systematic refactoring methodology (Fowler) | When improving existing code |
+| `test-design` | Dave Farley test design scoring (Farley Index) | Reviewing or writing tests |
+| `cognitive-load` | Cognitive complexity analysis (CLI score) | Identifying hotspots, before refactoring |
+| `problem-analysis` | Deep problem understanding before coding | Starting any non-trivial task |
+| `user-stories` | Elephant Carpaccio story slicing | Breaking down features |
+| `atdd` | Acceptance Test-Driven Development workflow | Feature development with acceptance criteria |
+| `spec-driven-dev` | Spec-driven development for PMs | Writing specs and requirements |
+| `planning` | Planning and task breakdown | Sprint planning, task decomposition |
+| `gitea-ci` | Gitea CI/CD pipeline with act_runner, buildah, k3s, GitHub mirror | Setting up or debugging CI on this infra |
+| `debug` | Hypothesis-first debugging discipline | Any test failure or bug whose cause is not immediately obvious |
+| `feature-spec` | Implementation-level feature spec (tighter than spec-driven-dev) | Scoping one feature inside an already-specced project |
+| `playwright` | Playwright E2E testing — setup, HTMX patterns, SSE streams, CI wiring | Adding browser tests to any project with a web UI |
+| `session-retrospective` | Surface non-obvious learnings from a session log | After a coding session ends, before context is lost |
+| `trainer` | Two-phase brain curation (score candidates, write past quality gate) | Periodic brain pruning and curation |
+| `grill-me` | Structured plan interrogation — Quick Poke, Full Grill, Pre-mortem | Stress-testing a plan before committing; end of Diamond 1; before promoting to pre-prod |
+
+## Wiring into tools
+
+The canonical home is `~/dev/.skills/`. Per-tool surfacing is handled by
+`~/dev/.context/wire-skills.sh`:
+
+- **Claude Code** — each git repo under `~/dev/` gets a `<repo>/.claude/skills`
+  symlink pointing at `~/dev/.skills/`. The symlink is excluded by
+  `**/.claude/skills` in `~/.config/git/ignore`, so it never lands in commits.
+- **Crush** — per-skill symlinks under `~/.config/crush/skills/<name>`, added
+  alongside existing entries (e.g., `mstack/*`). Skills already present as
+  real directories or non-canonical symlinks are preserved.
+
+Run `bash ~/dev/.context/wire-skills.sh --dry-run` to preview, then drop the
+flag to apply. The script is idempotent.
+
+## Skill Relationships
+
+```
+problem-analysis
+    │
+    └── user-stories
+            │
+            ├── spec-driven-dev (PM-kickoff lifecycle)
+            │       │
+            │       └── feature-spec (per-feature, mid-implementation)
+            │               │
+            │               └── planning
+            │                       │
+            │                       └── atdd ──→ tdd (foundation)
+            │                                ├──→ debug (when tests fail)
+            │                                ├──→ clean-code (REFACTOR phase)
+            │                                └──→ test-design (review)
+            │
+            └── planning
+
+code-review ──→ refactoring ──→ clean-code
+                                    └──→ solid
+
+(post-session)
+session-retrospective ──→ trainer ──→ brain (via brain_write)
+
+(planning & convergence)
+grill-me ──→ planning (after plan is sharpened)
+         ──→ spec-driven-dev (after hypothesis is validated)
+```
+
+## Quick Triggers
+
+| Situation | Load |
+|-----------|------|
+| "Build this feature" | `problem-analysis` → `spec-driven-dev` → `planning` → `atdd` |
+| "Fix this bug" | `tdd` |
+| "Review this code" | `code-review` |
+| "Refactor this" | `refactoring` + `clean-code` |
+| "Design this system" | `solid` + `problem-analysis` |
+| "Break this down into stories" | `user-stories` |
+| "Write a spec" | `spec-driven-dev` |
+| "This code is hard to understand" | `cognitive-load` |
+| "Review these tests" | `test-design` |
+| "Set up CI for this project" | `gitea-ci` |
+| "CI pipeline is failing / not triggering" | `gitea-ci` |
+| "Commit directly to main? Or branch?" | TBD: commit to main unless parallel agents are active (see `gitea-ci` SKILL §Trunk-Based Development) |
+| "Grill me / stress-test this / is this ready?" | `grill-me` |
+| "End of Diamond 1 — should we build this?" | `grill-me` (Full Grill) |
+| "About to promote to pre-prod" | `grill-me` (Pre-mortem) |