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.

CHANGELOG.md

@@ -0,0 +1,95 @@
+# Changelog
+
+All notable changes to patterm are tracked in this file. Format follows
+[Keep a Changelog](https://keepachangelog.com/en/1.1.0/) and the project
+loosely follows [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
+
+## [Unreleased]
+
+### Added
+- Ctrl+A / Ctrl+D step focus between top-level tabs; Ctrl+W / Ctrl+S
+  step through processes (root + sub-agents) inside the current tab.
+  Recognised in legacy, kitty CSI u, and xterm modifyOtherKeys
+  encodings. The chord shadows the corresponding raw byte for the
+  focused pane — pressing Ctrl+D no longer sends EOF to the
+  underlying shell, for instance.
+- MCP protocol layer (`internal/mcp/protocol.go`) implementing
+  `initialize`, `tools/list`, `tools/call`, `ping`, and MCP
+  notifications. Tool catalog with input schemas is advertised via
+  `tools/list`. Real MCP clients (claude, etc.) can now complete the
+  handshake against patterm's per-PID socket. Legacy direct-tool
+  dispatch is preserved so the harness keeps working unchanged.
+- `mcp_injection.kind = "cli_override"` for agents that accept inline
+  `key=value` config overrides on the command line. The default codex
+  preset uses it to emit `-c mcp_servers.patterm.command=…` and
+  `-c mcp_servers.patterm.args=[…]` — zero files written, no
+  `CODEX_HOME` override.
+- `mcp_injection.kind = "config_env"` for agents that read their
+  config from an env var. The default opencode preset uses it to pass
+  a merged `opencode.json` inline through `OPENCODE_CONFIG_CONTENT`,
+  so auth/agents/tui.json/skills resolve from the user's real `$HOME`
+  with no XDG override.
+- Palette macros: typing `sw `, `k `, or `sp ` filters the list to
+  switch / kill / spawn entries respectively. Footer shows the
+  available macros.
+
+### Changed
+- Palette ordering: open agents/processes (`Switch to …`) now appear
+  above the option to spawn new ones, with kill entries pushed down
+  toward the end of the list.
+- Tab bar trimmed from three rows (label / subtitle / underline) to
+  two (label / underline). Tabs flex to fill the available host width
+  evenly with leftover columns distributed to the leftmost tabs; the
+  `+ new` hint sits in a reserved slot on the right. Layout's
+  `mainTop` consequently drops from 4 to 3, giving each pane one
+  extra row of viewport.
+
+### Fixed
+- Killed agents no longer linger in the command palette. Agent
+  entries that aren't running are filtered out of the switch list;
+  session-persistent commands (which can be restarted) stay visible.
+- `tools/list` now emits a concrete `properties` object (`{}`) for
+  parameterless tools instead of `null`. Claude rejected the
+  `null`-properties form with "tools fetch failed" even though the
+  initialize handshake had succeeded.
+- Sidebar no longer flickers on every PTY chunk. The tab bar,
+  sidebar, and status line now cache their last rendered byte string
+  and skip the write when the new frame matches; full repaint paths
+  (resize / focus change / palette close / screen clear) invalidate
+  the cache so the next draw fires unconditionally.
+- Spawning a child agent now clears the viewport area before it
+  paints, so the previous focused child's PTY output no longer bleeds
+  through underneath the new pane.
+- Orchestrator-injected input (initial agent prompts, MCP
+  `send_input` with `submit: true`, `send_message`, `timer_wait`
+  callbacks) now ends with CR (`\r`) instead of LF (`\n`). Claude
+  treated `\n` as "newline in textarea"; with CR the prompt actually
+  submits, matching what the host terminal sends when a user presses
+  Enter directly.
+- Enter is now written to a child PTY as its own `write()` call,
+  separated from the preceding text by a short delay. Both
+  `InjectAsUser` (user typing forwarded through patterm) and
+  `InjectAsOrchestrator` (MCP / send_message / initial-prompt paths)
+  share the split. Without it, claude — and other paste-detecting
+  TUIs — coalesced `"hello\r"` into one read and inserted the CR as
+  literal text instead of treating it as the Enter keystroke.
+- Sidebar (and tab bar) no longer get wiped when the focused child
+  issues `CSI 0 J` / `CSI 1 J` (clear-to-cursor). The viewport renderer
+  already clamped `CSI 2 J` and `CSI K` to viewport columns, but the
+  partial-screen variants were forwarded verbatim, so any tool-call
+  expansion in claude (Ctrl+O) would erase every cell to the right of
+  the cursor — including the right rail. Both forms are now translated
+  into per-row ECH sequences that stop at the viewport's right edge.
+- Sidebar left border no longer vanishes when the viewport repaints.
+  The border column was the same column as the viewport's rightmost
+  cell, so any child write to that column (or `clearViewport`'s ECH)
+  would erase it. The viewport is now one column narrower so the
+  border has a dedicated column.
+
+## Conventions
+
+- This file is the single record of user-visible changes; the TODO is
+  scratch space, not history.
+- One bullet per change, written in the past tense from the user's
+  point of view. Reference the package or preset name when it helps a
+  reader find the code.