feat(pgpm): add pgpm ls for listing workspace modules

[pyramation]

Summary

Adds pgpm ls (alias list) so workspace module discovery is a CLI call instead of shell plumbing. Motivated by the boilerplate CI matrix, which had to discover packages with ls -d packages/*/ | xargs -n1 basename | jq -Rsc ...; that becomes:

- id: list
  run: echo "packages=$(pnpm exec pgpm ls --paths --json)" >> "$GITHUB_OUTPUT"

The command reads new PgpmPackage(cwd).getModuleMap() — so it reports actual pgpm modules (pgpm.plan/.control), not whatever directories happen to sit under packages/, and the paths it emits are workspace-relative.

pgpm ls                  # name  version  path, aligned, sorted by name
pgpm ls --json           # [{ name, version, path, requires }, ...] (pretty)
pgpm ls --names --json   # ["a","b"]           single line, for $(...) capture
pgpm ls --paths --json   # ["packages/a","packages/b"]
pgpm ls --names|--paths  # one per line, plain

Two behaviors worth calling out:

• --json mode writes nothing to stdout but the JSON, so CI can capture stdout verbatim.
• Outside a workspace it exits non-zero via cliExitWithError even with --json — a broken discover step must fail the job rather than silently yield []. An empty-but-valid workspace still prints [].

ls/list are registered in the engine-exempt set: the command never touches Postgres.

Consumed by constructive-io/pgpm-boilerplates#53.

Link to Devin session: https://app.devin.ai/sessions/dd4055c50bcf4b1e8e604211038aaa82
Open in Devin Desktop: https://app.devin.ai/desktop/session/dd4055c50bcf4b1e8e604211038aaa82?variant=devin
Requested by: @pyramation

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[tenki-reviewer]

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

🧹 Nitpicks (1) — 🟢 1 low

• 🟢 Missing trailing newline in --json --names/--paths (ls.ts:70) — With json set, the --names and --paths branches write JSON.stringify(...) without a trailing newline (pgpm/cli/src/commands/ls.ts:70, :74), while the full --json object branch appends one (:77) and other CLI JSON output uses console.log.

---

This PR introduces a new pgpm ls command in pgpm/cli/src/commands/ls.ts that discovers modules via workspace.getModuleMap(), sorts them by name, and emits them in four output modes (column-aligned text, --names, --paths, and --json), wiring it into the CLI router in commands.ts and index.ts, with a corresponding test suite in ls.test.ts.

Files	Change
pgpm/cli/src/commands/ls.ts	New command implementing module listing with text/names/paths/JSON output modes and flag-conflict handling
pgpm/cli/src/commands.ts, pgpm/cli/src/index.ts	Route and register the new ls command in the CLI
pgpm/cli/__tests__/ls.test.ts	Adds fixtures-based tests covering the different output modes and the empty-workspace path
.agents/skills/pgpm/references/cli.md	Documents the new ls command usage and options

_{Reviewed commit: 5bed508}