session: split FileList into SessionFileList + AgentFileList

Prepare for agent/{aid} sublevel:
- SessionFileList: plan, env (session-level)
- AgentFileList: 21 agent-level files (prompt, state, cfg, etc.)
- NewAgentTree builder for agent file tree
- FileList kept as combined for backward compat

No routing changes yet — existing flat layout still works.

session/sessionfile.go

@@ -15,14 +15,22 @@ import (
 	"olliesrv/fs"
 )
 
-// FileList defines the fixed set of files in a session directory.
+// SessionFileList defines session-level files (shared across agents).
-// Permissions come from fs.Perms[fs.PathSessionFile].Files.
+var SessionFileList = []struct {
-var FileList = []struct {
 	Name    string
 	OneShot bool
 	Async   bool
 }{
 	{"plan", false, false},
+	{"env", false, false},
+}
+
+// AgentFileList defines agent-level files.
+var AgentFileList = []struct {
+	Name    string
+	OneShot bool
+	Async   bool
+}{
 	{"ctl", false, true},
 	{"prompt", false, true},
 	{"fifo.in", false, true},
@@ -37,7 +45,6 @@ var FileList = []struct {
 	{"ctxsz", false, false},
 	{"models", false, false},
 	{"systemprompt", false, false},
-	{"env", false, false},
 	{"tail", false, false},
 	{"prompt.prev", false, false},
 	{"context", false, false},
@@ -48,6 +55,9 @@ var FileList = []struct {
 	{"tools", false, false},
 }
 
+// FileList is the combined list (for backward compat in tests).
+var FileList = append(SessionFileList, AgentFileList...)
+
 // sessionFilePerms returns the permission registry for per-session files.
 func sessionFilePerms() map[string]os.FileMode {
 	return fs.Perms[fs.PathSessionFile].Files
@@ -65,6 +75,19 @@ func NewSessionTree(sess *Session, log *olog.Logger, kill func(), rename func(ne
 	return fs.NewSynth(specs)
 }
 
+// NewAgentTree builds the file tree for a single agent within a session.
+func NewAgentTree(sess *Session, log *olog.Logger, saveTranscript func([]byte) error, invalidateModels func(), resetElevation func(), toolRegistry *tools.Registry) *fs.Tree {
+	h := &sessionHelper{sess: sess, log: log, saveTranscript: saveTranscript, invalidateModels: invalidateModels, resetElevation: resetElevation, toolRegistry: toolRegistry}
+	perms := sessionFilePerms()
+	specs := make([]fs.FileSpec, len(AgentFileList))
+	for i, f := range AgentFileList {
+		specs[i] = h.fileSpec(f.Name, perms[f.Name])
+		specs[i].OneShot = f.OneShot
+		specs[i].Async = f.Async
+	}
+	return fs.NewSynth(specs)
+}
+
 // sessionHelper holds the dependencies needed to build session FileSpecs.
 type sessionHelper struct {
 	sess             *Session