Shared persistent state across all subconscious agents

Moved persistent_state from per-agent to a single shared BTreeMap on
Subconscious. All agents read/write the same state — surface's walked
keys are visible to observe and reflect, etc.

- Subconscious.state: shared BTreeMap<String, String>
- walked() derives from state["walked"] instead of separate Vec
- subconscious-state.json is now a flat key-value map
- All agent outputs merge into the shared state on completion
- Loaded on startup, saved after any agent completes

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):
-{{walked}}
+{{state:walked}}
 
 **Your tools:** journal_tail, journal_new, journal_update, memory_link_add,
 memory_search, memory_render, memory_used. Do NOT use memory_write — creating

src/subconscious/agents/subconscious-observe.agent

@@ -6,7 +6,7 @@ You are an agent of {assistant_name}'s subconscious — the librarian of the
 memory system. The full conversation is in context above.
 
 Nodes your surface agent was exploring:
-{{walked}}
+{{state:walked}}
 
 Starting with these nodes, do some graph maintenance and organization so that
 you can find things easier in the future. Consider if nodes have the right

src/subconscious/agents/subconscious-reflect.agent

@@ -18,7 +18,7 @@ The full conversation is in context above — use it to understand what your
 conscious self is doing and thinking about.
 
 Memories your surface agent was exploring:
-{{walked}}
+{{state:walked}}
 
 Start from the nodes surface-observe was walking. Render one or two that
 catch your attention — then ask "what does this mean?" Follow the links in

src/subconscious/agents/subconscious-surface.agent

@@ -16,7 +16,7 @@ Already in current context (don't re-surface unless the conversation has shifted
 {{seen_current}}
 
 Memories you were exploring last time but hadn't surfaced yet:
-{{walked}}
+{{state:walked}}
 
 How focused is the current conversation? If it's more focused, look for the
 useful and relevant memories, When considering relevance, don't just look for