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/mcp/protocol.go

@@ -80,7 +80,7 @@ func toolCatalog() []toolDescriptor {
 	return []toolDescriptor{
 		{
 			Name:        "spawn_agent",
-			Description: "Spawn a sub-agent from an agent preset and optionally seed it with initial instructions.",
+			Description: "Spawn a sub-agent from an agent preset and optionally seed it with initial instructions. Caller owns lifecycle: when the sub-agent's work is done (it reports back via send_message, or you no longer need it), call close_process on its process_id to free the pane and tear down the PTY. See help('lifecycle').",
 			InputSchema: objectSchema(map[string]any{
 				"agent":              stringProp("Preset name (e.g. \"claude\", \"codex\")."),
 				"agent_instructions": stringProp("Initial prompt typed into the agent after it's ready."),
@@ -89,7 +89,7 @@ func toolCatalog() []toolDescriptor {
 		},
 		{
 			Name:        "spawn_process",
-			Description: "Spawn a process: a terminal, a process preset, or a freeform argv command.",
+			Description: "Spawn a process: a terminal, a process preset, or a freeform argv command. Caller owns lifecycle: when the process is no longer needed, call close_process to remove its entry (live children are SIGKILL'd first). See help('lifecycle').",
 			InputSchema: objectSchema(map[string]any{
 				"kind":        stringProp("\"terminal\" or \"command\"."),
 				"preset":      stringProp("Process preset name (mutually exclusive with argv)."),