AutoAgent: persistent across runs, run() vs run_forked()

AutoAgent holds config + walked state. Backend is ephemeral per run:
- run(): standalone, global API client (oneshot CLI)
- run_forked(): forks conscious agent, resolves prompt templates
  with current memory_keys and walked state

Mind creates AutoAgents once at startup, takes them out for spawned
tasks, puts them back on completion (preserving walked state).

Removes {{seen_previous}}, {{input:walked}}, {{memory_ratio}} from
subconscious agent prompts. Walked keys are now a Vec on AutoAgent,
resolved via {{walked}} from in-memory state.

Co-Authored-By: Proof of Concept <poc@bcachefs.org>

src/subconscious/agents/subconscious-journal.agent

@@ -6,7 +6,7 @@ The full conversation is in context above — use it to understand what your
 conscious self is doing and thinking about.
 
 Nodes your subconscious recently touched (for linking, not duplicating):
-{{input:walked}}
+{{walked}}
 
 **Your tools:** journal_tail, journal_new, journal_update, memory_link_add,
 memory_search, memory_render, memory_used. Do NOT use memory_write — creating