feat(pgpm): add slice command for modularizing migration plans

Summary

Adds a new pgpm slice command that partitions a large monolithic migration plan into multiple modular packages. This addresses the maintainability challenge of projects with thousands of migrations in a single package.

The implementation supports two grouping strategies:

1. Folder-based strategy (default): Groups changes by folder depth in the path (e.g., schemas/auth/tables/users → auth package)
2. Pattern-based strategy: Groups changes using glob patterns defined in a JSON file (e.g., schemas/*_auth_*/** → auth package)

Cross-package dependencies are expressed using package:change syntax, and the algorithm computes a valid deployment order via topological sort.

Key components:

• Core slicing logic (pgpm/core/src/slice/): Dependency graph building, DAG validation, package assignment, cycle detection, pattern matching via minimatch
• CLI command (pgpm slice): Supports --strategy, --patterns, --dry-run, configurable depth/prefix, and minimum package size
• Integration test (pgpm/core/__tests__/slice/slice-deploy-integration.test.ts): End-to-end test that slices plans and deploys packages to a real database
• Design spec (docs/spec/SLICING.md): Comprehensive documentation of the algorithm and edge cases

Updates since last revision

• Moved spec file from pgpm/MIGRATION_SLICING_SPEC.md to docs/spec/SLICING.md to follow repo conventions
• All CI checks passing (50/50)

Review & Testing Checklist for Human

• Test with a real plan file: Run pgpm slice --dry-run against an actual large plan file to verify grouping and dependency detection work correctly - unit tests may not catch real-world edge cases
• Verify cross-package dependency syntax: The code generates package:change format for cross-package deps - confirm this matches PGPM's expected resolution format
• Test pattern-based slicing: Create a JSON pattern file and run pgpm slice --strategy pattern --patterns ./slices.json --dry-run to verify glob matching works correctly
• Validate generated plan files: Ensure output plan files can be parsed back by parsePlanFile() without errors
• Review cycle handling: The algorithm warns but proceeds when package cycles are detected - verify this is acceptable behavior

Recommended Test Plan

1. Test folder-based strategy:

   pgpm slice --dry-run --plan <path>

2. Test pattern-based strategy with a pattern file:

   {
     "slices": [
       { "packageName": "auth", "patterns": ["schemas/*_auth_*/**"] },
       { "packageName": "users", "patterns": ["schemas/*_users_*/**"] }
     ]
   }

   pgpm slice --strategy pattern --patterns ./slices.json --dry-run

3. Run without --dry-run and inspect the generated packages directory
4. Verify each generated pgpm.plan file is valid and deployable

Notes

• Unit tests and integration tests pass covering core slicing functions including pattern matching and actual deployment
• The algorithm warns but proceeds when package cycles are detected (may want stricter handling)
• extensions/* and migrate/* paths are automatically assigned to core package
• Pattern matching uses first-match semantics (order of slices in JSON matters)

Devin run: https://app.devin.ai/sessions/84604c65fadd4983a065f855e6c82533
Previous session: https://app.devin.ai/sessions/e11be896d4dd45aaa746d1e2c42c186a
Requested by: @pyramation