docs: reorganize README into focused guides and add concord-database skill

Slim README from 3155 lines to 145 lines, extracting all detailed content
into dedicated docs/ guides for hex docs:
- getting-started.md: installation, quick start, common use cases
- elixir-guide.md: consistency levels, CAS, queries, compression, API ref
- observability.md: telemetry, prometheus, tracing, audit, event streaming
- backup-restore.md: CLI and programmatic backup/restore
- configuration.md: all config options and environment variables
- deployment.md: Docker, K8s, security, operations, FAQ, troubleshooting

Update mix.exs to include all new docs in hex docs with grouped navigation.

Add concord-database skill (skills/concord-database/) for AI agents with
SKILL.md core guide and references/ for API, state machine, and HTTP API.

Add /update-concord-database-skill command to refresh the skill from source.

Author: GSMLG-BOT

.claude/commands/update-concord-database-skill.md

@@ -0,0 +1,48 @@
+Perform a fresh, comprehensive analysis of the Concord project and update the concord-database skill at `skills/concord-database/`.
+
+## Analysis Steps
+
+1. **Read all source files** to capture the current state of the codebase:
+   - `lib/concord.ex` — All public API functions, options, return types
+   - `lib/concord/state_machine.ex` — All commands, queries, state shape, snapshot logic
+   - `lib/concord/auth.ex` — Auth API
+   - `lib/concord/rbac.ex` — RBAC API
+   - `lib/concord/multi_tenancy.ex` — Multi-tenancy API
+   - `lib/concord/index.ex` and `lib/concord/index/extractor.ex` — Index API and extractor specs
+   - `lib/concord/backup.ex` — Backup/restore API
+   - `lib/concord/ttl.ex` — TTL management
+   - `lib/concord/query.ex` — Query language API
+   - `lib/concord/event_stream.ex` — Event streaming API
+   - `lib/concord/compression.ex` — Compression API
+   - `lib/concord/web/router.ex` and `lib/concord/web/authenticated_router.ex` — HTTP routes
+   - `lib/concord/web/api_controller.ex` — HTTP handler details
+   - `lib/concord/application.ex` — Supervisor tree
+   - `config/config.exs` — All configuration options
+   - `config/runtime.exs` — Runtime configuration
+   - `CLAUDE.md` — Correctness invariants and architecture
+   - `mix.exs` — Dependencies and aliases
+
+2. **Compare with current skill** — Read existing skill files:
+   - `skills/concord-database/SKILL.md`
+   - `skills/concord-database/references/api-reference.md`
+   - `skills/concord-database/references/state-machine.md`
+   - `skills/concord-database/references/http-api.md`
+
+3. **Update all skill files** to reflect any changes:
+   - New/removed/changed API functions
+   - New/changed state machine commands or queries
+   - New/changed HTTP endpoints
+   - Updated configuration options
+   - New modules or features
+   - Updated correctness invariants
+   - Changed testing patterns
+
+## Guidelines
+
+- Keep SKILL.md concise (<500 lines) — it shares context window space
+- Use imperative/infinitive form in instructions
+- Include only information an AI agent wouldn't already know
+- Ensure all code examples match actual source code signatures
+- Verify correctness invariants still match CLAUDE.md
+- Update references/ files with detailed information
+- Do NOT create README.md or other auxiliary files