P3-X2: tree.list synchronous WS RPC + per-session cache

Foundational for the restore wizard's tree browser. The wizard needs to
lazy-load directory contents from a snapshot as the operator drills
down; this lands the transport.

- internal/api adds MsgTreeList (server → agent) + MsgTreeListResult
  (agent → server) with TreeListRequestPayload / TreeListEntry /
  TreeListResultPayload types. Reply correlates by Envelope.ID.
- internal/restic.ListTreeChildren wraps 'restic ls --json' and
  filters its recursive output to direct children of the requested
  path. Parser + path-normalisation + isDirectChild are unit-tested.
- internal/server/ws/rpc.go introduces a generic SendRPC helper on
  Hub: register a buffered channel keyed by ULID, send the request,
  block on ctx.Done()/timeout/reply. Reply routing piggybacks on the
  existing dispatchAgentMessage by adding a MsgTreeListResult case
  that forwards to the registered waiter; if no waiter is registered
  (caller already gave up) the stray reply is dropped quietly.
- cmd/agent gains a tree.list handler that runs ListTreeChildren on a
  fresh per-call context (60s ceiling) and ships the matching
  tree.list.result envelope. Errors surface in result.Error rather
  than as transport failures so the server-side waiter can render a
  sensible UI message.
- internal/server/http/tree_cache.go is the per-wizard-session cache
  layer (~30min TTL, sweep-on-access) that fetchTreeWithCache uses
  before falling through to SendRPC. Cached on success only; agent
  errors aren't cached so a transient failure doesn't poison the
  session.

Tests:
- internal/restic/ls_test.go covers parseLsChildren at root / mid-tree
  / leaf, plus normalizeTreePath and isDirectChild edge cases.
- internal/server/ws/rpc_test.go unit-tests the registry: round-trip,
  release semantics, concurrent waiters, ctx-cancel.
- internal/server/http/tree_rpc_test.go is the full round-trip: server
  SendRPC → fake-agent over a real WS → reply → server gets the
  payload. Plus a timeout test that confirms ~300ms timeouts terminate
  in ~300ms rather than waiting forever.

The cache is plumbed but no UI handler hits fetchTreeWithCache yet —
that lands with P3-01 (wizard backend). The unused-linter is suppressed
via nolint until the wizard wires it in.

internal/api/wire.go

@@ -12,18 +12,19 @@ type MessageType string
 
 // Agent → server message types.
 const (
-	MsgHello         MessageType = "hello"
-	MsgHeartbeat     MessageType = "heartbeat"
-	MsgJobStarted    MessageType = "job.started"
-	MsgJobProgress   MessageType = "job.progress"
-	MsgJobFinished   MessageType = "job.finished"
-	MsgSnapshotsRpt  MessageType = "snapshots.report"
-	MsgRepoStats     MessageType = "repo.stats"
-	MsgLogStream     MessageType = "log.stream"
-	MsgScheduleAck   MessageType = "schedule.ack"
-	MsgScheduleFire  MessageType = "schedule.fire"  // agent: a local cron entry fired, please dispatch a job
-	MsgCommandResult MessageType = "command.result" // ack for command.run
-	MsgError         MessageType = "error"
+	MsgHello          MessageType = "hello"
+	MsgHeartbeat      MessageType = "heartbeat"
+	MsgJobStarted     MessageType = "job.started"
+	MsgJobProgress    MessageType = "job.progress"
+	MsgJobFinished    MessageType = "job.finished"
+	MsgSnapshotsRpt   MessageType = "snapshots.report"
+	MsgRepoStats      MessageType = "repo.stats"
+	MsgLogStream      MessageType = "log.stream"
+	MsgScheduleAck    MessageType = "schedule.ack"
+	MsgScheduleFire   MessageType = "schedule.fire"    // agent: a local cron entry fired, please dispatch a job
+	MsgCommandResult  MessageType = "command.result"   // ack for command.run
+	MsgTreeListResult MessageType = "tree.list.result" // reply to a server-driven tree.list
+	MsgError          MessageType = "error"
 )
 
 // Server → agent message types.
@@ -33,6 +34,7 @@ const (
 	MsgScheduleSet      MessageType = "schedule.set"
 	MsgConfigUpdate     MessageType = "config.update"
 	MsgAgentUpdateAvail MessageType = "agent.update.available"
+	MsgTreeList         MessageType = "tree.list" // sync RPC: list a snapshot's children
 )
 
 // Envelope is the framing for every WS message in either direction.