feat(pgpm): pgpm doctor — OS-aware dependency checks; robust docker daemon detection

[pyramation]

Summary

Turns the quickstart prerequisites page into an executable check and makes pgpm docker fail with actionable, OS-specific guidance. No install-time automation — doctor only runs when invoked.

• New pgpm doctor [--db] [--json] (pgpm/cli/src/commands/doctor.ts): checks node (fail <18, warn <20), docker (binary on PATH AND docker info daemon reachability — distinguishes "not installed" from "installed but daemon down"), docker compose (plugin or standalone, warn-only), psql (fail if missing, warn <17), and with --db a live SELECT 1 via psql using getPgEnvOptions()/getSpawnEnvWithPg(). Exit 1 on any failure; --json emits { platform, checks, summary } for CI.
• Shared checks module pgpm/cli/src/utils/doctor.ts:
  • detectPlatform() → macos | linux(+distro from /etc/os-release) | wsl(/proc/version) | windows
  • dockerInstallGuidance / dockerDaemonGuidance / psqlInstallGuidance return per-OS remediation (brew vs apt-get vs dnf vs winget; "open Docker Desktop" vs systemctl start docker + docker-group hint)
  • all checks accept an injectable CommandRunner for testing
• pgpm docker hardened: checkDockerAvailable() (binary-only) replaced with ensureDockerReady() — start/stop now report daemon-down vs not-installed with the same OS-aware guidance; docker ls requires the daemon, not just the binary.
• 25 unit tests (pgpm/cli/__tests__/doctor.test.ts) with mocked runners covering platform detection, each check's pass/warn/fail paths, and per-OS guidance.

Plan: https://github.com/constructive-io/constructive-planning/issues/1258 (PR 1 of the rollout; init-time checks and docs deferred).

Link to Devin session: https://app.devin.ai/sessions/3810d68dc3384dc5b4430a63da7af981
Requested by: @pyramation

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