demote plan from session level to agent level

plan now lives at session/{id}/agent/{aid}/plan instead of
session/{id}/plan. Moved the Leaf declaration from fs/sessionfiles.go
to fs/agentfiles.go. Updated both prompt docs to match.

data/prompts/agent-coding.md

@@ -5,7 +5,7 @@ You are a coding agent.
 - Before non-trivial tasks: inspect, plan, act, verify, update plan.
 - Verify means: run the build, run affected tests. Not "looks right to me".
 - Keep plans short, explicit, and task-focused.
-- Use `${OLLIE}/session/${OLLIE_SESSION_ID}/plan` for multi-step work.
+- Use `${OLLIE}/session/${OLLIE_SESSION_ID}/agent/${OLLIE_AGENT_NAME}/plan` for multi-step work.
 - Update the plan after major progress, blockers, or changes in approach.
 
 # Constraints

data/prompts/session-9p.md

@@ -77,7 +77,6 @@ Sessions live at `session/`. Each session is a directory; all operations are 9P
 | `models`      | read  | Available models for current backend |
 | `offset`      | read  | Byte offset after last user prompt |
 | `peer/`       | dir   | Peer links. `create` to add, `remove` to remove (both sides updated). Write to send prompt to peer. |
-| `plan`        | r/w   | Session-scoped markdown checklist; survives compaction; visible to peers |
 | `proc/`       | dir   | Detached background processes |
 | `prompt`      | write | Submit a prompt to the agent |
 | `prompt.prev` | read  | Previous prompt text |
@@ -87,6 +86,14 @@ Sessions live at `session/`. Each session is a directory; all operations are 9P
 | `tail`        | exec  | Tail helper |
 | `usage`       | read  | Token usage stats |
 
+### Agent Directory (`session/{id}/agent/{aid}/`)
+
+Each agent in a session gets its own subdirectory with its own plan, state, and chat log.
+
+| File | Mode | Purpose |
+|---|---|---|
+| `plan`        | r/w   | Markdown checklist; survives compaction; visible to peers |
+
 ## Operations
 
 ```bash

fs/agentfiles.go

@@ -168,6 +168,24 @@ func listAgents(ctx HandlerCtx) ([]FsNodeDecl, error) {
 				return []byte(fmt.Sprintf("%d\n", al.ChatOffset)), nil
 			})),
 
+			// plan — agent plan (markdown checklist, survives compaction)
+			Leaf("plan", 0666,
+				Read(func(_ HandlerCtx) ([]byte, error) {
+					al.mu.RLock()
+					data := make([]byte, len(al.plan))
+					copy(data, al.plan)
+					al.mu.RUnlock()
+					return data, nil
+				}),
+				Write(func(_ HandlerCtx, data []byte) error {
+					al.mu.Lock()
+					al.plan = make([]byte, len(data))
+					copy(al.plan, data)
+					al.mu.Unlock()
+					return nil
+				}),
+			),
+
 			// cfg — agent config (backend, model, etc.)
 			Leaf("cfg", 0666,
 				Read(func(_ HandlerCtx) ([]byte, error) {

fs/sessionfiles.go

@@ -42,32 +42,6 @@ func listSessions(ctx HandlerCtx) ([]FsNodeDecl, error) {
 				}),
 			),
 
-			// plan — session plan
-			Leaf("plan", 0666,
-				Read(func(_ HandlerCtx) ([]byte, error) {
-					al := s.AgentLog()
-					if al == nil {
-						return nil, nil
-					}
-					al.mu.RLock()
-					data := make([]byte, len(al.plan))
-					copy(data, al.plan)
-					al.mu.RUnlock()
-					return data, nil
-				}),
-				Write(func(_ HandlerCtx, data []byte) error {
-					al := s.AgentLog()
-					if al == nil {
-						return nil
-					}
-					al.mu.Lock()
-					al.plan = make([]byte, len(data))
-					copy(al.plan, data)
-					al.mu.Unlock()
-					return nil
-				}),
-			),
-
 			// name — mutable session name
 			Leaf("name", 0666,
 				Read(func(_ HandlerCtx) ([]byte, error) {