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/mcp/mcp.go

@@ -115,9 +115,11 @@ func (s *Server) handleConn(conn net.Conn) {
 	} else {
 		// Treat as a real request from an unknown caller.
 		resp := s.dispatch("", greeting)
-		resp = append(resp, '\n')
-		if _, werr := conn.Write(resp); werr != nil {
-			return
+		if resp != nil {
+			resp = append(resp, '\n')
+			if _, werr := conn.Write(resp); werr != nil {
+				return
+			}
 		}
 	}
 
@@ -125,9 +127,11 @@ func (s *Server) handleConn(conn net.Conn) {
 		line, err := r.ReadBytes('\n')
 		if len(line) > 0 {
 			resp := s.dispatch(callerID, line)
-			resp = append(resp, '\n')
-			if _, werr := conn.Write(resp); werr != nil {
-				return
+			if resp != nil {
+				resp = append(resp, '\n')
+				if _, werr := conn.Write(resp); werr != nil {
+					return
+				}
 			}
 		}
 		if err != nil {