feat(pgpm): zero-config extensions cleanup + --pglite init flag (consolidated)

[pyramation]

Summary

Consolidates three PRs (#1354, #1356, #1357) into one. Together they finish the extensions-workflow cleanup and add the --pglite init flag. Pairs with the docs consolidation in constructive-io/constructive.io.

1. Harden pgpm extension (was #1356)

Makes pgpm extension the canonical enable-later path:

• Non-destructive .control rewrite — edits only the requires = line, preserving custom fields (schema, comment, relocatable, module_pathname, …) instead of regenerating the file from defaults.
• Non-interactive flags --add/--remove/--set for CI/scripts; no flags ⇒ interactive picker (unchanged).
• Snapshots updated to reflect that custom fields now survive a dependency change (the desired behavior).

2. --pglite init flag with workspace inheritance (was #1354)

--pglite is sugar over --repo pointing at pglite-boilerplates:

pgpm init workspace --pglite   # records boilerplates:{repo} in workspace pgpm.json
pgpm init                      # inside that workspace → inherits pglite, no flag
pgpm init --pglite | --repo …  # explicit flags still override

Resolve/persist/read helpers live in pgpm/cli/src/commands/init/boilerplate.ts (network-free unit tests); new PGLITE_TEMPLATE_REPO constant and optional PgpmWorkspaceConfig.boilerplates type.

3. pgpm skill docs (was #1357)

Updates .agents/skills/pgpm/ to document zero-config init + the non-interactive pgpm extension flags, and corrects the now-false "fully interactive — don't use in CI" claims.

(PR A, #1355 — defaulting pgpm init to no extensions — already merged and is the base these build on.)

Tests

All three branches' suites merged cleanly (no conflicts): extension writer/command unit + integration tests, init flag resolution + gated e2e, updated snapshots. 37/37 green on each source PR.

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

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