Add sd-docs-writer agent and /sd:adr command - #12

What

Implements the Planned roadmap item sd-docs-writer agent. Adds a 6th agent and an 11th command
that draft a numbered, MADR-style Architecture Decision Record under .specs/_adr/ from a spec's
decision artifacts, behind one hard approval gate.

Changes

• agents/docs-writer.md (new, sd-docs-writer) - model sonnet, minimal tools
  Read/Write/Glob/Grep, skill sd-evidence-citation. Drafts ONE ADR from a decision source; never
  edits the constitution, never modifies code, never invents decisions.
• commands/adr.md (new, /sd:adr <spec-ID | "title">) - Phase 0 bootstrap, resolves the decision
  source (spec 03-decisions.md or ad-hoc), assigns the next zero-padded ADR number + kebab slug,
  invokes the agent, gates HARD before keeping, and handles supersession links.
• Count wiring 10 -> 11 / 5 -> 6 across scripts/validate.{ps1,sh} (hardcoded constants),
  CLAUDE.md, README.md, docs/architecture.md (routing tree + agent table + evidence-citation
  consumers), install/README.md (incl. total 33 -> 35), CONTRIBUTING.md, commands/setup.md
  report, and templates/CLAUDE.template.md.
• CHANGELOG.md - [Unreleased] -> Added entry.
• docs/superpowers/ - design doc + implementation plan for traceability.

Design decisions

• ADRs only - tightest scope the constitution already references (.specs/_adr/); architecture-doc
  generation and a close-gate offer are deferred follow-ups.
• Dedicated command, not a workflow hook - one new entry point instead of branching four existing
  workflow files.
• Agent writes only the ADR file; the command owns numbering and the superseded-ADR reciprocal link
  (scope discipline, mirrors sd-implementer).

Verification

• scripts/validate.ps1 - all 6 checks pass (Check 5: commands/sd : 11, agents/sd : 6; Check 4:
  6 agents use model aliases).
• Sandbox install round-trip: 11 commands / 6 agents land, adr.md + docs-writer.md present, uninstall
  removes commands/sd cleanly.

Note on merge order

Overlaps commands/setup.md and CHANGELOG.md with #11 (setup codebase scan). Whichever merges second
will need a small rebase + CHANGELOG resolve - the documented sequential-merge pattern.

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