v0.5.2

Closes #12. Two features: whole-setup diagnostics, and making the write preview mandatory.

Breaking / Behaviour changes

• execute_write_query now defaults to dry_run=true, and the preview is enforced rather
  than advisory. A commit (dry_run=false) is rejected unless the identical statement was
  dry-run first. A bare call therefore previews instead of committing — an agent that used to
  call the tool and have it write will now get a preview back.

  The grant fingerprints database + SQL + params with a 10-minute TTL. skip_dry_run=true
  exists solely for an agent to attest the user explicitly asked to skip the preview.

  Gate order is now mode → dry-run-first → yolo → user_consent. yolo cannot skip the
  preview, and nothing can ever make a read database writable.

Added

• doctor — whole-setup diagnostics, not just connectivity. One engine (doctor.py), two
  surfaces: db-conn-mcp doctor [--offline] (exit 0 only if nothing fails, 2 otherwise) and a
  doctor MCP tool returning {check, status, detail, suggested_action} so an agent can
  self-diagnose mid-session. 23 tools total.

  Six checks, each drawn from a real failure during the 0.5.0 → 0.5.1 upgrade: running server
  processes older than the installed package (psutil optional, and it reports its own host
  process when stale); a cache-bypassed PyPI version check; per-database connectivity plus a
  credential-free listener probe of fallback ports, catching "a different local Postgres
  answered my port"; config-schema typos with did-you-mean hints; secrets exposure (POSIX file
  mode, git-committability); and injected client entries whose command path no longer exists.

  The engine never raises — a crashing check degrades to a fail naming only the exception
  type. A poisoned-DSN sweep test asserts no DSN, host, user or password can reach any finding,
  including via pydantic validation errors (Rule 6).

• check_database UNREACHABLE results now report failed_port.

Changed

• MCP-client helpers moved from cli.py into a new clients.py (re-exported for
  compatibility), unlocking reuse without a circular import.

Full Changelog: v0.5.1...v0.5.2