Land staged session/MCP/chrome work + sidebar clear-J fix

This batches the in-flight [Unreleased] block from CHANGELOG.md into a
single commit. Highlights:

- Real MCP protocol layer (initialize / tools/list / tools/call) so
  vendor MCP clients can complete the handshake against the per-PID
  socket. Legacy direct-dispatch preserved for the harness.
- New mcp_injection kinds — cli_override for codex, config_env for
  opencode — joining the existing env-var and config_file paths so
  patterm can slot into more agents without touching their real
  config or auth.
- Ctrl+A/D and Ctrl+W/S focus navigation across tabs and intra-tab
  process lists, recognised in legacy / kitty CSI u / xterm
  modifyOtherKeys encodings.
- Palette macros (sw / k / sp ) and reordering so open sessions
  surface above spawn-new entries.
- Two-row tab bar, sidebar/tabbar/status chrome cache, viewport-wipe
  on agent spawn, CR-terminated orchestrator injections, and split-
  Enter PTY writes so paste-detecting TUIs see Enter as a key event.

Also fixes the bug logged in TODO: claude's Ctrl+O tool-call expansion
emits CSI 0 J, which the viewport renderer was forwarding verbatim —
wiping the sidebar to the right of the cursor and leaving the chrome
cache convinced nothing had changed. CSI 0 J and CSI 1 J are now
translated into per-row ECH sequences clamped to the viewport, same
as CSI 2 J and CSI K already were.

Agent guides (CLAUDE.md / AGENTS.md) now spell out the
TODO->CHANGELOG workflow so completed items land in the changelog
rather than as ticked entries left behind in TODO.

internal/app/sidebar.go

@@ -152,8 +152,17 @@ func (st *uiState) drawSidebar() {
 		write("")
 	}
 
+	frame := b.String()
+	st.chromeCacheMu.Lock()
+	if frame == st.sidebarCache {
+		st.chromeCacheMu.Unlock()
+		return
+	}
+	st.sidebarCache = frame
+	st.chromeCacheMu.Unlock()
+
 	st.outMu.Lock()
 	// Save cursor; emit the sidebar; restore.
-	fmt.Fprintf(os.Stdout, "\x1b7%s\x1b8", b.String())
+	fmt.Fprintf(os.Stdout, "\x1b7%s\x1b8", frame)
 	st.outMu.Unlock()
 }