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.

CLAUDE.md

@@ -95,6 +95,29 @@ PATTERM_BIN=/absolute/path/to/patterm go test ./internal/harness/...
 
 Without `PATTERM_BIN`, harness tests build the current checkout once into a temp location and test that binary.
 
+## Changelog
+
+User-visible changes go in `CHANGELOG.md` (Keep-a-Changelog format).
+When finishing work that affects users — new MCP tools, palette
+behavior, preset shapes, host chrome, anything observable — add a
+bullet under `[Unreleased]` in the appropriate `Added` / `Changed` /
+`Fixed` / `Removed` section. The TODO file is scratch space, not
+history; the changelog is the record.
+
+When a `TODO.md` item is actioned (bug fixed, behavior changed,
+feature shipped), the resolution belongs in `CHANGELOG.md` — not as
+a "done" entry left in `TODO.md`. Workflow:
+
+1. Land the code change.
+2. Add a `[Unreleased]` bullet in `CHANGELOG.md` describing what the
+   user will now experience differently.
+3. Remove the corresponding item from `TODO.md` (don't tick it off
+   and leave it behind — `TODO.md` only lists outstanding work).
+
+If a TODO item turns out to be a non-issue or gets dropped without a
+code change, just delete it from `TODO.md`; no changelog entry is
+needed.
+
 ## Development Notes
 
 - Prefer existing package boundaries. MCP protocol shapes live in `internal/mcp`; runtime behavior usually belongs in `internal/app`.