feat(workflows): pipeline pass rates and skill/plugin usage

Restores the five analytics sections that turned out to be worth keeping
(patterns, model delegation, error propagation, session complexity, compaction
impact) and reworks what the page opens on.

The lane panel no longer mirrors the Workspace's live lane strip. It now
answers the question the strip cannot: how often each pipeline node actually
passes. GET /api/lanes/pipeline-stats aggregates every stage record of every
lane AND every archived feature — a lane's own `stages` only holds its current
feature, so without the archive the history resets each time a lane starts new
work. Evidence-backed passes are counted apart from bare claims, the same line
the pipeline map draws between `done` and `passed-no-evidence`.

Beside it, GET /api/workflows/skill-usage reports which skills and plugins the
sessions actually reach for. Only PreToolUse rows are counted: every call also
emits a PostToolUse, and counting both would double every number. A plugin's
total includes calls to the MCP tools it ships, so one used purely through MCP
does not read as unused.

Still dropped from issue #6: the agent collaboration network and the subagent
effectiveness scorecards.

client/README.md

@@ -178,7 +178,7 @@ client/
 │   │   ├── Sidebar.tsx
 │   │   ├── Layout.tsx
 │   │   ├── RemoteSources.tsx  # Remote Data Sources settings panel (SSH multi-machine collection)
-│   │   └── workflows/      # Workflows page panels: lane pipelines + D3 analytics (6 files)
+│   │   └── workflows/      # Workflows page panels: own-pipeline stats + D3 analytics (12 files)
 │   │
 │   ├── pages/              # Route pages
 │   │   ├── Dashboard.tsx      # Monitor / Analytics / Health tabs