fix(snapshots): snapshot running VMs disk-only over QMP

[NovusEdge]

Snapshots of a running local VM failed because QEMU refuses savevm while the always-on work virtfs share is mounted. The share stays, since copy_to and copy_from depend on it.

• A running VM now takes a disk-only internal snapshot with blockdev-snapshot-internal-sync on the qcow2 device found through query-block. It records no RAM and is crash-consistent; stoat has no guest agent to freeze filesystems. Delete uses blockdev-snapshot-delete-internal-sync. Command names and arguments checked against the QEMU 11.1 QMP reference.
• Restore refuses a running VM (already_running, "stop the VM first (stoat down )"), because QEMU cannot revert a live disk.
• Listing reads ImageInfo (qemu-img info --output=json when stopped, query-block when running). SIZE is the saved RAM state, so it shows "-" for disk-only snapshots. QEMU records no size for the disk part.
• Wire: Snapshot has created (RFC 3339 UTC) and size_bytes; the display strings are removed. VM has shared_dir and shared_mount.
• MCP: list_snapshots (read) and delete_snapshot (destructive, like restore). copy_to, copy_from and destroy descriptions name the shared directory; copy errors name the host path.
• docs/reference/cli.md no longer shows a RAM=yes snapshot of a running VM.

Tests use a fake QMP socket and the real qemu-img on a scratch qcow2; no VM is booted. Live verification is still needed on a running VM.

[coderabbitai]

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📥 Commits

Reviewing files that changed from the base of the PR and between ba413a3 and 5f56d03.

📒 Files selected for processing (18)

• docs/reference/cli.md
• docs/reference/json.md
• docs/reference/mcp.md
• internal/cli/display_test.go
• internal/cli/run_state.go
• internal/cli/wire/dto.go
• internal/cli/wire/dto_test.go
• internal/core/snapshot.go
• internal/core/snapshot_test.go
• internal/core/vm.go
• internal/mcpsrv/redact_test.go
• internal/mcpsrv/table_test.go
• internal/mcpsrv/tools_guest.go
• internal/mcpsrv/tools_vm.go
• internal/qemu/qmp.go
• internal/qemu/qmp_test.go
• internal/tui/snapshots.go
• internal/tui/snapshots_test.go

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