Add stopped-workspace disk export and inspection

Finish the 3.1.0 secondary disk-tools milestone so stable workspaces can be
stopped, inspected offline, exported as raw ext4 images, and started again
without changing the primary workspace-first interaction model.

Add workspace stop/start plus workspace disk export/list/read across the CLI,
SDK, and MCP, backed by a new offline debugfs inspection helper and guest-only
validation. Scrub runtime-only guest state before disk inspection/export, and
fix the real guest reliability gaps by flushing the filesystem on stop and
removing stale Firecracker socket files before restart.

Update the docs, examples, changelog, and roadmap to mark 3.1.0 done, and
cover the new lifecycle/disk paths with API, CLI, manager, contract, and
package-surface tests.

Validation: uv lock; UV_CACHE_DIR=.uv-cache make check; UV_CACHE_DIR=.uv-cache
make dist-check; real guest-backed smoke for create, shell/service activity,
stop, workspace disk list/read/export, start, exec, and delete.

docs/first-run.md

@@ -22,7 +22,7 @@ Networking: tun=yes ip_forward=yes
 
 ```bash
 $ uvx --from pyro-mcp pyro env list
-Catalog version: 3.0.0
+Catalog version: 3.1.0
 debian:12 [installed|not installed] Debian 12 environment with Git preinstalled for common agent workflows.
 debian:12-base [installed|not installed] Minimal Debian 12 environment for shell and core Unix tooling.
 debian:12-build [installed|not installed] Debian 12 environment with Git and common build tools preinstalled.
@@ -81,6 +81,11 @@ $ uvx --from pyro-mcp pyro workspace snapshot create "$WORKSPACE_ID" checkpoint
 $ uvx --from pyro-mcp pyro workspace service start "$WORKSPACE_ID" web --ready-file .web-ready -- sh -lc 'touch .web-ready && while true; do sleep 60; done'
 $ uvx --from pyro-mcp pyro workspace reset "$WORKSPACE_ID" --snapshot checkpoint
 $ uvx --from pyro-mcp pyro workspace export "$WORKSPACE_ID" note.txt --output ./note.txt
+$ uvx --from pyro-mcp pyro workspace stop "$WORKSPACE_ID"
+$ uvx --from pyro-mcp pyro workspace disk list "$WORKSPACE_ID"
+$ uvx --from pyro-mcp pyro workspace disk read "$WORKSPACE_ID" note.txt
+$ uvx --from pyro-mcp pyro workspace disk export "$WORKSPACE_ID" --output ./workspace.ext4
+$ uvx --from pyro-mcp pyro workspace start "$WORKSPACE_ID"
 $ uvx --from pyro-mcp pyro workspace delete "$WORKSPACE_ID"
 ```
 
@@ -200,12 +205,33 @@ $ uvx --from pyro-mcp pyro workspace service stop WORKSPACE_ID web
 
 $ uvx --from pyro-mcp pyro workspace service stop WORKSPACE_ID worker
 [workspace-service-stop] workspace_id=... service=worker state=stopped execution_mode=guest_vsock
+
+$ uvx --from pyro-mcp pyro workspace stop WORKSPACE_ID
+Workspace ID: ...
+State: stopped
+
+$ uvx --from pyro-mcp pyro workspace disk list WORKSPACE_ID src --recursive
+Workspace: ...
+Path: /workspace/src
+- /workspace/src [directory]
+- /workspace/src/note.txt [file] bytes=...
+
+$ uvx --from pyro-mcp pyro workspace disk read WORKSPACE_ID src/note.txt
+hello from synced workspace
+[workspace-disk-read] workspace_id=... path=/workspace/src/note.txt size_bytes=... truncated=False
+
+$ uvx --from pyro-mcp pyro workspace disk export WORKSPACE_ID --output ./workspace.ext4
+[workspace-disk-export] workspace_id=... output_path=... disk_format=ext4 bytes_written=...
+
+$ uvx --from pyro-mcp pyro workspace start WORKSPACE_ID
+Workspace ID: ...
+State: started
 ```
 
 Use `--seed-path` when the workspace should start from a host directory or a local
 `.tar` / `.tar.gz` / `.tgz` archive instead of an empty `/workspace`. Use
 `pyro workspace sync push` when you need to import later host-side changes into a started
-workspace. Sync is non-atomic in `3.0.0`; if it fails partway through, prefer `pyro workspace reset`
+workspace. Sync is non-atomic in `3.1.0`; if it fails partway through, prefer `pyro workspace reset`
 to recover from `baseline` or one named snapshot. Use `pyro workspace diff` to compare the current
 `/workspace` tree to its immutable create-time baseline, `pyro workspace snapshot *` to create
 named checkpoints, and `pyro workspace export` to copy one changed file or directory back to the
@@ -219,7 +245,9 @@ service must be reachable from the host on `127.0.0.1`. Use `--secret` and `--se
 workspace creation when the sandbox needs private tokens or config. Persisted secret files are
 materialized at `/run/pyro-secrets/<name>`, and `--secret-env SECRET_NAME[=ENV_VAR]` maps one
 secret into one exec, shell, or service call without storing that environment mapping on the
-workspace itself.
+workspace itself. Use `pyro workspace stop` plus `pyro workspace disk list|read|export` when you
+need offline inspection or one raw ext4 copy from a stopped guest-backed workspace, then
+`pyro workspace start` to resume the same workspace.
 
 The stable workspace walkthrough GIF in the README is rendered from
 [docs/assets/workspace-first-run.tape](assets/workspace-first-run.tape) with