Update README, LLM setup guide, and bundled wt skill for orchestration workflows

[pkudinov]

Parent: #11

Implementation dependencies:

• Support database-optional worktree lifecycle #13
• Make slot allocation and registry mutation concurrency-safe #14
• Define a stable, versioned JSON contract for orchestration #15

Goal

Update all user-facing and agent-facing guidance together after the underlying
contracts land. This is intentionally one documentation issue so configuration,
CLI examples, automation guidance, and the bundled /wt skill cannot drift
apart.

README updates

Product positioning and requirements

• Describe databases as an optional isolation capability rather than a global
  prerequisite.
• Make the PostgreSQL requirement conditional on managed template-cloning mode.
• Explain the three database states represented by an allocation: managed,
  logical-name-only, and absent.
• State what new, setup, remove, prune, and doctor do in each state.

Configuration examples

• Preserve and verify the existing local Postgres template-cloning example.
• Add a ports + env-only example that has no local database or DATABASE_URL.
• Add a cloud/pre-provisioned database example that disables provisioning but
  derives a per-slot logical DB name for the database env patch.
• Document validation behavior for {{dbName}} and database patches when no
  logical name is configured.

Multi-service cross-wiring

• Add one complete example with at least a frontend and API service.
• Show the API's PORT using a port patch.
• Show the frontend's API_BASE_URL using a url patch targeting the API
  service, proving both values resolve to the same allocated port.
• Include the resulting env values for a concrete slot.

Concurrent orchestration

• Document the exact slot/port reservation guarantee from Make slot allocation and registry mutation concurrency-safe #14.
• Explain explicit-slot conflicts, stale reservations, crash recovery, and the
  relevant doctor/prune behavior.
• Include a safe fan-out example for multiple concurrent wt new --json
  processes.

JSON contract

• Document the schema version and compatibility policy from Define a stable, versioned JSON contract for orchestration #15.
• Provide complete success and error examples for new --json.
• Provide a complete list --json example with resolved ports.
• State that stdout contains the JSON document and diagnostics go to stderr.

Setup Guide for LLM Agents

• Make database discovery conditional instead of assuming every project has a
  Postgres DATABASE_URL.
• Add an explicit decision step: managed local Postgres, externally managed
  database with logical per-slot naming, or no database integration.
• Teach agents to generate the implemented configuration shape for each mode.
• Keep port and URL discovery service-aware so cross-service URLs remain
  self-consistent.
• Add mode-appropriate smoke tests that do not create or drop a database in
  unmanaged mode.
• Teach agents to use the versioned JSON fields rather than scraping terminal
  output.

Bundled skills/wt/SKILL.md

• Mirror the LLM setup decision tree and all validation rules.
• Update routing, examples, requirements, smoke tests, and troubleshooting for
  database-optional operation.
• Include the cross-service port + url mapping example.
• Describe concurrency and JSON guarantees precisely, without promising
  behavior beyond Make slot allocation and registry mutation concurrency-safe #14 and Define a stable, versioned JSON contract for orchestration #15.

Acceptance criteria

• README configuration, command reference, requirements, patch-type reference,
  registry example, JSON section, and LLM setup guide all use the final
  implemented names and shapes.
• skills/wt/SKILL.md matches the README and contains no unconditional
  Postgres assumptions.
• Examples are exercised against the CLI or corresponding contract tests.
• Links between Feature request: DB-optional mode — use port + env isolation without a local Postgres (for parallel multi-agent worktrees) #11 and Support database-optional worktree lifecycle #13–Define a stable, versioned JSON contract for orchestration #15 make the implementation dependencies explicit.
• Documentation does not introduce a pluggable provisioner or provider-specific
  setup that is outside the accepted implementation scope.