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.

internal/app/app.go

@@ -330,6 +330,18 @@ func (st *uiState) OnChildSpawned(c *Child) {
 		st.palette.rebuild()
 		st.renderPaletteLocked()
 	}
+	// Prime the snapshot-replay budget for the new child. Diff-based
+	// vendor TUIs (claude/codex/opencode) emit incremental updates that
+	// assume the host display already matches their internal "last
+	// frame" model. On a fresh spawn the host viewport was just cleared,
+	// so incremental ops target cells that aren't populated yet —
+	// leaving the corrupted pane the user works around by toggling
+	// focus (which routes through repaintFocused). Setting the budget
+	// here makes the next ~8 PTY chunks render from the full styled
+	// emulator grid, so the host display tracks the emulator state
+	// without needing a manual focus cycle.
+	st.repaintNextPTY = c.ID
+	st.repaintNextPTYBudget = 8
 	st.mu.Unlock()
 
 	// Wipe the viewport area so the previous focused child's PTY