feat: Add lattice diff command for branch-scoped change summaries

.lattice/implementations/cli-019.yaml

@@ -0,0 +1,19 @@
+id: IMP-CLI-019
+type: implementation
+title: lattice diff command implementation
+body: Implements branch-scoped diff via git diff --name-status. Core logic in src/diff.rs, CLI wiring in src/main.rs. Supports text, JSON, and markdown output modes.
+status: active
+version: 1.0.0
+created_at: 2026-03-08T18:15:58.019437+00:00
+created_by: agent:claude-2026-03-08
+requested_by: George Moon <george.moon@gmail.com>
+meta:
+  files:
+  - path: src/diff.rs
+  - path: src/main.rs
+  - path: src/lib.rs
+  - path: tests/diff_test.rs
+edges:
+  satisfies:
+  - target: REQ-CLI-019
+    version: 1.0.0

.lattice/requirements/cli/019-branch-scoped-lattice-diff-command.yaml

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+id: REQ-CLI-019
+type: requirement
+title: Branch-scoped lattice diff command
+body: 'The CLI provides a ''lattice diff'' command that shows lattice nodes added, modified, or resolved since a given git ref. Defaults to merge-base with main. Supports --since for explicit ref, --md for markdown output suitable for GitHub comments, and --format json for structured output. Groups changes by type: added, modified, resolved, deleted.'
+status: active
+version: 1.0.0
+created_at: 2026-03-08T18:15:47.317389+00:00
+created_by: agent:claude
+requested_by: George Moon <george.moon@gmail.com>
+priority: P1
+category: CLI
+tags:
+- cli
+- diff
+- agent-workflow
+resolution:
+  status: verified
+  resolved_at: 2026-03-08T18:16:01.109674+00:00
+  resolved_by: agent:claude-2026-03-08
+edges: {}