Split the preset agent module by concern

agent.py had grown to 1591 lines mixing six concerns. Split it, moving every
symbol verbatim:

- session.py: on-disk session state, loaders, ownership claims, liveness
- workspace.py: the agent working directory, aliasing, wrappers, skills
- tail.py: offset-persistent tailers over stream and record files
- redaction.py: secret redaction shared by the tailers and verify
- agent.py: the Claude subprocess lifecycle (600 lines)

Imports follow one direction: session <- workspace <- (tail, redaction) <-
agent. No behavior change; all symbol bodies are byte-identical to before.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: Andrey Cheptsov

src/dstack/_internal/cli/commands/preset.py

@@ -13,12 +13,6 @@
     PresetListOutput,
 )
 from dstack._internal.cli.services.completion import ProjectNameCompleter
-from dstack._internal.cli.services.presets.agent import (
-    find_session_name_claims,
-    get_presets_dir,
-    list_agent_sessions,
-    load_resumable_agent_session,
-)
 from dstack._internal.cli.services.presets.apply import apply_preset
 from dstack._internal.cli.services.presets.create import (
     create_preset,
@@ -28,6 +22,12 @@
     stop_preset_session,
 )
 from dstack._internal.cli.services.presets.output import print_presets
+from dstack._internal.cli.services.presets.session import (
+    find_session_name_claims,
+    get_presets_dir,
+    list_agent_sessions,
+    load_resumable_agent_session,
+)
 from dstack._internal.cli.services.presets.store import (
     PresetStore,
     load_preset_configuration,