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

[pyramation]

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 based on folder structure. This addresses the maintainability challenge of projects with thousands of migrations in a single package.

The implementation uses a path-based algorithm where change names like schemas/auth/tables/users are grouped into packages (e.g., auth). 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, and output generation
• CLI command (pgpm slice): Supports --dry-run, configurable depth, prefix stripping, and minimum package size
• Design spec (pgpm/MIGRATION_SLICING_SPEC.md): Comprehensive documentation of the algorithm and edge cases

Review & Testing Checklist for Human

• Verify cross-package dependency syntax: The code generates package:change format for cross-package deps - confirm this matches PGPM's expected resolution format
• 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
• Validate generated plan files: Ensure output plan files can be parsed back by parsePlanFile() without errors
• Check deploy order correctness: For complex dependency chains, verify packages are ordered correctly (dependencies before dependents)
• Test CLI manually: Run pgpm slice --help and try slicing a sample plan to verify the command works end-to-end

Recommended Test Plan

1. Find or create a plan file with 10+ changes across multiple schemas
2. Run pgpm slice --dry-run --plan <path> and review the output
3. Run without --dry-run and inspect the generated packages/ directory
4. Verify each generated pgpm.plan file is valid

Notes

• 18 unit tests pass covering core slicing functions
• The algorithm warns but proceeds when package cycles are detected (may want stricter handling)
• extensions/* and migrate/* paths are automatically assigned to core package

Devin run: https://app.devin.ai/sessions/e11be896d4dd45aaa746d1e2c42c186a
Requested by: @pyramation

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