chore(claude): add the engineering toolkit with tiered model pins

[mhenrixon]

What

Adds a .claude/ engineering toolkit and a top-level CLAUDE.md so autonomous
and semi-autonomous workflows (/lfg and its specialists) run against docs-kit's
own domain — the DocsUI:: Phlex kit, the per-site config surface, the registry,
the install generator, and the CSS/deploy contracts.

Every command and agent pins a model tier alias in frontmatter rather than a
full model ID, so pins track the latest model per tier and never go stale.

Tier	Where	Rationale
fable	/plan	Read-only planning: investigates, designs, emits a self-contained issue or docs/plans/ markdown for cheaper models to execute
opus	/lfg, /architect, /security, /review-pr, /github-review-pr	Orchestration, security, PR review
sonnet	/tdd, /github-review-comments, /github-review-failures	Pattern-following implementation with prescriptive prompts
haiku	mechanical subagents (documented in rules/agents.md)	File finding, pattern scans

Why

• A pinned full model ID (claude-opus-4-8) goes stale on every model release;
  tier aliases (opus/sonnet/haiku/fable) fix that class of staleness
  permanently by tracking the latest model per tier.
• Forcing the most expensive model on every turn is wasteful — layer specialists
  and the review runbooks follow prescriptive prompts and run well a tier down.
• /plan puts the capable model where it pays off: it audits and writes a
  self-contained spec (context, ordered steps, verification gates, out-of-scope)
  that /lfg executes on a cheaper model without any of the planning session's
  context.

What's included

• Commands (.claude/commands/): plan, lfg, architect, security,
  review-pr, github-review-pr, github-review-failures,
  github-review-comments, tdd.
• Rules (.claude/rules/): coding-style, git-workflow, testing,
  agents.
• CLAUDE.md: critical rules, architecture map, slash-command table, and the
  model-tier convention.
• .claude/README.md: toolkit index + tier convention.
• .claude/settings.local.json: permission allowlist.

Notes

• Content is written for docs-kit invariants: compose from DocsUI:: components
  (never raw daisyUI), read DocsKit.configuration with defaults (backwards
  compat), progressive enhancement (works with JS off), one docs-nav Stimulus
  controller, config.themes in sync with the Tailwind @plugin list, and
  required setup wired into the install generator + docs-kit new template.
• A perf command/rule was intentionally left out — docs-kit has no benchmark
  harness.

Test plan

• All 9 commands carry a tier alias in frontmatter
• No stray references to unrelated projects' internals
• .claude/ is tracked (not gitignored)

Docs/config only — no application code, no specs affected.