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>

internal/daemon/ARCHITECTURE.md

@@ -32,13 +32,11 @@ owning types:
 - `createOps opstate.Registry[*vmCreateOperationState]` — in-flight VM
   create operations; owns its own lock.
 - `tapPool tapPool` — TAP interface pool; owns its own lock.
-- `sessions sessionRegistry` — active guest session controllers; owns
-  its own lock.
 - `listener`, `vmDNS` — networking.
 - `vmCaps` — registered VM capability hooks.
 - `pullAndFlatten`, `finalizePulledRootfs`, `bundleFetch`,
   `requestHandler`, `guestWaitForSSH`, `guestDial`,
-  `waitForGuestSessionReady` — injectable seams used by tests.
+  `workspaceInspectRepo`, `workspaceImport` — injectable seams used by tests.
 
 ## Subpackages
 
@@ -53,11 +51,9 @@ state beyond small test seams.
 | `internal/daemon/dmsnap`          | Device-mapper COW snapshot create/cleanup/remove.                      |
 | `internal/daemon/fcproc`          | Firecracker process primitives (bridge, tap, binary, PID, kill, wait). |
 | `internal/daemon/imagemgr`        | Image subsystem pure helpers: validators, staging, build script gen.   |
-| `internal/daemon/session`         | Guest-session helpers: state paths, scripts, parsing, utilities.       |
 | `internal/daemon/workspace`       | Workspace helpers: git inspection, copy prep, guest import script.     |
 
-`workspace` imports `session` for `ShellQuote` and `FormatStepError`; all
-other subpackages are leaves (no other intra-daemon subpackage imports).
+All subpackages are leaves — no intra-daemon subpackage imports another.
 
 ## Lock ordering
 
@@ -73,9 +69,8 @@ time. `workspace.prepare` acquires `vmLocks[id]` just long enough to
 validate VM state, releases it, then acquires `workspaceLocks[id]`
 for the guest I/O phase.
 
-Subsystem-local locks (`tapPool.mu`, `sessionRegistry.mu`,
-`opstate.Registry` mu, `guestSessionController.attachMu` /
-`writeMu`) are leaves. They do not contend with each other.
+Subsystem-local locks (`tapPool.mu`, `opstate.Registry` mu) are leaves.
+They do not contend with each other.
 
 Notes: