feat(pgpm): change-granularity 'single' level + pgpm-projections skill

[pyramation]

Summary

Completes the change-granularity dial to three symmetric levels (mirrors statement granularity's atomic/object/consolidated), stacked on #1585:

--change-granularity alteration | object | single

• single: the whole restructured module collapses into one plan entry (module/init by default; singleChangeName option) with one deploy/revert/verify. Requires collapse to only the input set's external deps (cross-module project:change refs survive; internal edges vanish). Implemented as an early branch in restructureChanges — deploy is the full topological script, revert/verify come from revertFor/verifyFor over all statements.
• pgpm transform / pgpm diff / pgpm import accept the new value automatically via CHANGE_GRANULARITIES (usage text updated).
• New skill .agents/skills/pgpm-projections/SKILL.md: documents the full projections pipeline — the four dials (granularity, change granularity, naming, partition), output projections (module / linear SQL / bundle), library seams, and the invariants (semantic identity + catalog equivalence across every dial combination).

Tests: unit (collapse, custom name, external-deps-only), live-Postgres transform-e2e (single-entry plan deploys to the identical catalog, verifies, reverts clean), and examples/pgpm-projections now asserts single-mode semantic invariance alongside alteration.

Link to Devin session: https://app.devin.ai/sessions/798445577ac8487abf80e28f9ff7a916
Requested by: @pyramation

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