docs(pgpm): skill accepts a developer's own Postgres; never stop what owns 5432

[pyramation]

Summary

Docs-only (replaces #1804, which also changed pgpm env; that part is dropped — if you export your own PG* vars you simply don't run pgpm env).

The pgpm skill told agents that "port 5432 in use" means stop the process, which during onboarding meant killing the developer's own PostgreSQL. Four .agents/skills/pgpm files now say instead:

• pg_isready -h localhost -p 5432 first. If it answers, that's the developer's Postgres: use it with their credentials (export PGHOST/PGPORT/PGUSER/PGPASSWORD, pgpm admin-users bootstrap --yes), skip pgpm docker start and pgpm env, and never stop it.
• Want the container too? pgpm docker start --port 5433 + export PGPORT=5433.
• Only consider stopping something on 5432 if lsof shows it isn't Postgres.
• New troubleshooting row for ERR_PNPM_IGNORED_BUILDS: edit pnpm-policy.yaml (allowBuilds true/false) and pnpm run policy; don't pnpm approve-builds.

Companion PRs: constructive-io/pgpm-boilerplates#45, constructive-io/dev-utils#128.

Link to Devin session: https://app.devin.ai/sessions/1ef0d1c209f041afa29e0e4cc4b2cb29
Open in Devin Desktop: https://app.devin.ai/desktop/session/1ef0d1c209f041afa29e0e4cc4b2cb29?variant=devin
Requested by: @pyramation

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[tenki-reviewer]

Review complete. No blocking issues — approved ✅; 1 nitpick below.

🧹 Nitpicks (1) — 🟢 1 low

• 🟢 Truncated comment on bootstrap roles (troubleshooting.md:277) — The inline comment on pgpm admin-users bootstrap --yes at .agents/skills/pgpm/references/troubleshooting.md:277 reads # once: roles pgpm and pgsql-test expect and is cut off mid-sentence, so a reader cannot tell what the command is supposed to do.

---

This PR revises the pgpm skill docs to steer users toward an existing local PostgreSQL server rather than Docker. It adds a "use your own Postgres" path across the skill, covering how to export PGHOST/PGPORT/PGUSER/PGPASSWORD, run pgpm admin-users bootstrap --yes, and start the pgpm container on an alternate port (--port 5433) when both are wanted, while warning against stopping a developer's server.

Files	Change
.agents/skills/pgpm/SKILL.md	Adds the reuse-an-existing-server guidance to the install/start steps and the troubleshooting quick-fix table.
.agents/skills/pgpm/references/docker.md	Notes Docker is optional and documents the alternate-port container path.
.agents/skills/pgpm/references/env.md	Adds a "Using Your Own PostgreSQL" section and clarifies pgpm env emits container vars.
.agents/skills/pgpm/references/troubleshooting.md	Reworks the port-in-use guidance to prefer an existing Postgres and adds the bootstrap-role note.

_{Reviewed commit: c58a924}