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/LaneCard.tsx

@@ -363,19 +363,22 @@ export function LaneToolsMenu({ lane }: { lane: Lane }) {
           role="menu"
           className="absolute right-0 z-10 mt-1 min-w-44 overflow-hidden rounded-md border border-border-light bg-surface-0 py-1 shadow-lg shadow-black/40"
         >
-          <button
-            type="button"
-            role="menuitem"
-            data-testid="lane-agents-install"
-            onClick={(e) => {
-              e.stopPropagation();
-              setToolsMenuOpen(false);
-              void handleAgentsInstall();
-            }}
-            className="block w-full px-3 py-1.5 text-left text-fg-secondary hover:bg-surface-2"
-          >
-            {t("actions.agentsInstall")}
-          </button>
+          {/* Claude-only: a codex lane has no agents to install. */}
+          {lane.tool !== "codex" && (
+            <button
+              type="button"
+              role="menuitem"
+              data-testid="lane-agents-install"
+              onClick={(e) => {
+                e.stopPropagation();
+                setToolsMenuOpen(false);
+                void handleAgentsInstall();
+              }}
+              className="block w-full px-3 py-1.5 text-left text-fg-secondary hover:bg-surface-2"
+            >
+              {t("actions.agentsInstall")}
+            </button>
+          )}
           {lane.tool !== "codex" && (
             <button
               type="button"