remove vm session feature

Cuts the daemon-managed guest-session machinery (start/list/show/
logs/stop/kill/attach/send). The feature shipped aimed at agent-
orchestration workflows (programmatic stdin piping into a long-lived
guest process) that aren't driving any concrete user today, and the
~2.3K LOC of daemon surface area — attach bridge, FIFO keepalive,
controller registry, sessionstream framing, SQLite persistence — was
locking in an API we'd have to keep through v0.1.0.

Anything session-flavoured that people actually need today can be
done with `vm ssh + tmux` or `vm run -- cmd`.

Deleted:
- internal/cli/commands_vm_session.go
- internal/daemon/{guest_sessions,session_lifecycle,session_attach,session_stream,session_controller}.go
- internal/daemon/session/ (guest-session helpers package)
- internal/sessionstream/ (framing package)
- internal/daemon/guest_sessions_test.go
- internal/store/guest_session_test.go
- GuestSession* types from internal/{api,model}
- Store UpsertGuestSession/GetGuestSession/ListGuestSessionsByVM/DeleteGuestSession + scanner helpers
- guest.session.* RPC dispatch entries
- 5 CLI session tests, 2 completion tests, 2 printer tests

Extracted:
- ShellQuote + FormatStepError lifted to internal/daemon/workspace/util.go
  (only non-session consumer); workspace package now self-contained
- internal/daemon/guest_ssh.go keeps guestSSHClient + dialGuest +
  waitForGuestSSH — still used by workspace prepare/export
- internal/daemon/fake_firecracker_test.go preserves the test helper
  that used to live in guest_sessions_test.go

Store schema: CREATE TABLE guest_sessions and its column migrations
removed. Existing dev DBs keep an orphan table (harmless, pre-v0.1.0).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

docs/advanced.md

@@ -60,33 +60,19 @@ disk, or pass `--kernel /abs/path/vmlinux` for a one-off kernel.
 For reproducible custom images, write a Dockerfile and publish it to
 an image catalog. See [`docs/image-catalog.md`](image-catalog.md).
 
-## Workspace + session primitives
+## Workspace primitive
 
-Long-lived guest commands managed by the daemon, attachable over a
-local Unix socket bridge. Useful for agent/background processes that
-need to survive SSH disconnects.
+`vm run ./repo` (see README) handles the common case. For a manual
+flow against an already-running VM, `vm workspace prepare`
+materialises a local git checkout into the guest:
 
 ```bash
 banger vm workspace prepare <vm> ./other-repo --guest-path /root/repo
-banger vm session start <vm> --name planner --cwd /root/repo \
-  --stdin-mode pipe -- pi --mode rpc
-banger vm session attach <vm> planner
-banger vm session logs <vm> planner --stream stderr
-banger vm session stop <vm> planner
 ```
 
-Details:
-
-- `vm workspace prepare` materialises a local git checkout into a
-  running VM. Default guest path `/root/repo`; default mode is a
-  shallow metadata copy plus tracked and untracked non-ignored
-  overlay.
-- `vm session start` launches a daemon-managed long-lived guest
-  command. The daemon preflights that the guest `cwd` exists and the
-  command is on guest `PATH` before launch. Use `--stdin-mode pipe`
-  when you need live `attach`.
-- `vm session attach` is exclusive and same-host only. Pipe-mode
-  sessions survive daemon restarts.
+Default guest path is `/root/repo`; default mode is a shallow metadata
+copy plus tracked and untracked non-ignored overlay. For repositories
+with submodules, pass `--mode full_copy`.
 
 ## Inspecting boot failures