feat: throw error on duplicate module names in workspace

[pyramation]

Summary

Adds a DUPLICATE_MODULE error that is thrown when multiple modules in a pgpm workspace share the same .control file name, similar to how pnpm/yarn workspaces reject duplicate package names.

Before: getModules() silently returned all duplicates (causing duplicate entries in pgpm deploy's interactive prompt), and listModules() silently picked the shortest path.

After: Both methods throw a DUPLICATE_MODULE error listing the conflicting paths.

Two files changed:

• pgpm/types/src/error-factory.ts — new DUPLICATE_MODULE error definition
• pgpm/core/src/core/class/pgpm.ts — duplicate detection in getModules() and listModules()

Review & Testing Checklist for Human

• listModules() behavior change: The old code intentionally handled naming collisions by picking the shortest path (lines 334-343 before). This PR replaces that with a hard error. Verify this is the desired behavior — any workspace with legitimately duplicated .control file names (e.g. nested node_modules, symlinks) will now fail instead of deduplicating gracefully.
• Breaking change scope: Any existing workspace that has duplicate module names will start failing on any command that calls getModules() or listModules(). Confirm no existing workspaces besides agentic-db are affected, or that this is acceptable.
• getModuleName() reliability in getModules(): The new code calls proj.getModuleName() on each PgpmPackage instance. Verify this works correctly for all module types (the original code didn't need the module name, just isInModule()).
• No unit tests added — consider whether test coverage for the new error path is needed.
• Test manually: Set up a workspace with two directories containing .control files that share the same base name, run pgpm deploy, and confirm the error message is clear and actionable.

Notes

• Only the first duplicate group triggers the error (the for loop throws immediately). Users with multiple duplicate groups will need to fix them one at a time.
• The error uses HTTP code 400 consistent with other validation errors in the factory.

Link to Devin session: https://app.devin.ai/sessions/c26111a26a9a44f891534104a809dcdb
Requested by: @pyramation

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