feat: add cgc diff — CLI command to show graph changes between index snapshots #1312

Problem

There is currently no way to understand what changed in the code graph between two points in time — e.g. before and after a PR, or after a large refactor. Users must re-query manually to spot differences.

Issue #690 proposes a UI-level PR diff visualization, but there is no CLI-level tool for developers who want a quick, scriptable summary of graph changes.

Proposed solution

Add a cgc diff command that compares the current index state against a saved snapshot.

Example usage

cgc snapshot save --name before-refactor
# ... make code changes and re-index ...
cgc diff --against before-refactor

Example output

cgc diff — comparing current index vs snapshot "before-refactor"

+ FUNCTION  payment_service.validate_card         (new)
- FUNCTION  payment_service.check_card            (removed)
~ CALLS     checkout.process → payment_service.*  (target changed)
+ CLASS     billing.InvoiceBuilder                (new)
  5 nodes added · 2 nodes removed · 3 edges changed

Scope

• cgc snapshot save [--name ] — serialise the current graph state as a lightweight snapshot (node/edge manifest, not full data)
• cgc snapshot list — list saved snapshots
• cgc diff [--against ] — print added/removed/changed nodes and edges
• Optional --json flag for CI/scripting use
• Snapshots stored in ~/.codegraphcontext/snapshots/

Why this is useful

• PR reviews: see exactly what functions/classes a PR adds or removes before merging
• Refactor validation: confirm a rename touched every expected call site
• CI integration: fail a pipeline if unexpected functions were deleted
• Watch mode complement: understand accumulated changes since the last full index

Related issues

• Add special PR indexing: Snapshot comparison and code diff visualization #690 (UI-level PR diff — separate scope)
• [Feature] Git hook integration — auto-rebuild graph on commit (cgc hook install/uninstall) #886 (git hook integration — complementary)