feat(git): commit graph in history view and cross-spawn everywhere

- render a VSCode-style commit graph in the source control history:
  lane-assignment algorithm, SVG strip with colored rails, merge and
  branch curves, commit dots, and branch/tag badges per commit
- /commits returns parents and ref decorations across all branches
  (--branches --remotes --tags --topo-order) using unit-separator
  fields so pipes in commit subjects can't break parsing
- collect stats via a single --shortstat pass instead of one
  `git show --stat` call per commit; raise the history limit to 50
- replace child_process spawn with cross-spawn in all runtimes,
  routes, and services: it resolves .cmd shims and PATHEXT on
  Windows (fixing taskmaster's npx invocations) and delegates to
  native spawn elsewhere, removing the per-file win32 ternaries
- unit tests for the log parser and lane assignment

server/utils/plugin-process-manager.js

@@ -1,5 +1,7 @@
-import { spawn } from 'child_process';
 import path from 'path';
+
+// cross-spawn: drop-in spawn with Windows .cmd/PATHEXT resolution.
+import spawn from 'cross-spawn';
 import { scanPlugins, getPluginsConfig, getPluginDir } from './plugin-loader.js';
 
 // Map<pluginName, { process, port }>