Teach parent agents to clean up the processes they spawn

Add a `lifecycle` help topic spelling out that the caller owns the
processes it spawns and should `close_process` when a sub-agent or
spawned child is no longer needed. The `spawn_agent` and `spawn_process`
descriptions advertised via `tools/list` now restate the same duty
inline (with a pointer to `help('lifecycle')`), so vendor TUIs see the
expectation at the moment they reach for the tool. The `spawning` topic
and `topics` index cross-reference the new content.

Bundles two already-staged improvements that fall in the same area:
- OnChildSpawned primes the snapshot-replay budget for new panes so
  diff-based vendor TUIs come up clean without a manual Ctrl+W/Ctrl+S
  refresh.
- TODO drops the three items now actioned (prompt-injection preface,
  agent cleanup duty, opencode→claude view corruption) and keeps the
  unicode `<?>` entry with the investigation notes.

CHANGELOG.md

@@ -6,7 +6,28 @@ loosely follows [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
 ## [Unreleased]
 
+### Fixed
+- Sub-agent panes spawned while another diff-based TUI (claude/codex/
+  opencode) held focus could come up corrupted because the new child's
+  first incremental updates targeted cells the host viewport hadn't
+  populated yet. `OnChildSpawned` now primes the snapshot-replay budget
+  for the new child, so its first PTY chunks render from the full
+  styled emulator grid — the same path that fixed the symptom when the
+  user manually cycled focus with Ctrl+W / Ctrl+S.
+
 ### Added
+- New `lifecycle` help topic spelling out that the caller owns the
+  processes it spawns and should call `close_process` when a sub-agent
+  or spawned process is no longer needed. The `spawn_agent` and
+  `spawn_process` tool descriptions advertised via `tools/list` now
+  call out the same cleanup duty (with a pointer to `help('lifecycle')`),
+  and the `spawning` help topic and `topics` index cross-reference it.
+- `spawn_agent` now prepends a single-line `[system: …]` orientation
+  block to the sub-agent's first prompt so vendor TUIs (claude, codex,
+  opencode) know they're a sub-agent, are reminded to `send_message`
+  their parent with a summary when done, and to clean up processes /
+  scratchpads they created. Skipped for top-level launches and empty
+  instructions.
 - 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