feat: per-change-block hunk actions (#50), codex lanes without a pipeline, macOS LaunchAgent

- git (#50): stage-hunk/discard-hunk take an optional `block` — one run of
  consecutive +/- lines inside an @@ hunk. `onlyBlock` (server/lib/git-diff.js)
  cuts git's fresh hunk down to it, folding the other blocks into the side
  being checked; LaneDiffView hangs one review bar per block (unified + split)
  and its arrows walk blocks.
- git exclude: entries AgentHub writes for a lane go to the COMMON git dir with
  the lane's path prefix (server/lib/git-exclude.js, used by router-apply and
  lane-mcp) — a linked worktree's own info/exclude is never read by git, and a
  subdir lane has no .git. MCP sync no longer fails with ENOTDIR on a worktree
  lane or creates a stray .git in a non-repo lane.
- codex lanes carry no pipeline: codex-ingest mirrors only idle/running,
  codex-nudge is removed, agents install writes nothing for codex, the
  Workspace hides stage/pipeline/feature pickers/PipelineMap for them, and
  every server start clears leftover codex stage data and the old AGENTS.md
  stage block.
- Changes panel: a whole file opens for reading and enters the editor on a
  click on its text (caret at the clicked line), not automatically.
- macOS: LaunchAgent start-at-login (scripts/launchd-service.js, picked by
  scripts/service-manager.js).
- Adds the AgentHub introduction slides and their image-embedding script.

client/src/components/lanes/LaneStripCard.tsx

@@ -115,15 +115,18 @@ export default function LaneStripCard({
         <span className="shrink-0 text-[10px] font-semibold tracking-widest text-fg-muted">
           #{lane.id}
         </span>
-        <span className="truncate rounded bg-surface-4 px-1.5 py-0.5 text-fg-secondary">
-          {lane.stage}
-        </span>
-        {lane.tool === "codex" && (
+        {/* Codex lanes carry no pipeline, so no stage chip — the Codex tag
+            takes its place. */}
+        {lane.tool === "codex" ? (
           <span className="shrink-0 rounded border border-border bg-surface-2 px-1.5 py-0.5 text-[10px] text-fg-muted">
             Codex
           </span>
+        ) : (
+          <span className="truncate rounded bg-surface-4 px-1.5 py-0.5 text-fg-secondary">
+            {lane.stage}
+          </span>
         )}
-        {detectionLeads(lane) && (
+        {lane.tool !== "codex" && detectionLeads(lane) && (
           <span
             data-testid={`lane-tile-auto-${lane.id}`}
             title={lane.detected_signal || undefined}