docs(pgpm skill): document zero-config init + non-interactive pgpm extension

[pyramation]

Summary

Brings the pgpm agent skill in line with the new extensions workflow (default-no-extensions pgpm init + hardened pgpm extension). The most important fix is a correctness one: the skill previously told agents that pgpm init and pgpm extension are "fully interactive — don't use in CI", which is now false.

Changes across .agents/skills/pgpm/:

• SKILL.md: quickstart no longer says "select extensions"; the .control section notes a fresh module has no requires line; the command table and the CI-safety section now describe the non-interactive paths:
  • pgpm init --moduleName <module> [--extensions a,b] (no --extensions ⇒ zero extensions; init workspace uses --name)
  • pgpm extension --add/--remove/--set a,b (no flag ⇒ interactive picker)
• references/extensions.md, references/cli.md, references/dependencies.md, references/workspace.md: document the --add/--remove/--set flags, note that pgpm extension rewrites only the requires line (preserving custom .control fields), and drop the legacy requires = 'uuid-ossp,plpgsql' examples in favor of pgcrypto/module deps.

Docs-only; no code changes. Pairs with the tutorial refresh in constructive-io/constructive.io#14.

Link to Devin session: https://app.devin.ai/sessions/424b76f81a60499493ac946b57136ad9
Requested by: @pyramation

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[devin-ai-integration]

Consolidated into #1358 per request to reduce open PR count. Same commits, merged into one branch.