Add context listing and switching to the TUI

Press c to list cv4pve-cli contexts and switch between them. The
active context is always shown in the header.

- Switching runs `cv4pve-cli config use`, which changes the global
  current context. cv4pve-cli 2.4.0 has no per-call context option.
- `config list` has no JSON output, so its text lines are parsed.
- The guest table is cleared as soon as another context is chosen, so
  the old cluster's guests cannot be acted on.
- A generation counter drops guest listings that started before a
  switch, and g and c are refused while a switch runs. Without this,
  a reload already in flight could refill the table with the old
  cluster's guests, and VMIDs collide across clusters.

103 tests pass. The switch was tested live from mainframe to
mainframe-edge and back.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/plans/guest-tui-mvp.md

@@ -116,3 +116,66 @@ sockets/vCPU/hotplug settings, live auto-refresh.
 ## Open questions
 
 None blocking.
+
+---
+
+# Context switching
+
+## Problem
+
+Several cv4pve-cli contexts (clusters) exist; switch between them inside the TUI
+instead of quitting and running `cv4pve-cli config use`.
+
+## Decision
+
+Option A: switching runs `cv4pve-cli config use <name>`, which changes the **global**
+current context (all shells and scripts follow) and stays after the app exits. cv4pve-cli
+2.4.0 has no per-call context flag or env var (verified), so there is no app-local option.
+
+Observed CLI behaviour:
+
+- `config list` has no `-o json`. Text output, one line per context; current one
+  prefixed with `* `, others with two spaces:
+  `* mainframe            mainframe.iot.home.arpa:8006   (api-token)`
+  `  mainframe-edge       mainframe-edge.home.arpa:8006   (api-token)`
+- `config use <name>` -> exit 0 on success; unknown name -> exit 3,
+  `Error: Context '<name>' not found.`, current context unchanged.
+
+### `cli.py`
+
+- `contexts() -> list[dict]` - `run("config", "list")`, each line parsed to
+  `{"name", "host", "auth", "current": bool}` (regex
+  `^(\*)?\s*(\S+)\s+(\S+)\s+\((.*)\)\s*$`); non-matching lines are ignored.
+- `use_context(name)` - `run("config", "use", name)`.
+
+### `app.py` (UI contract)
+
+- The active context is always visible: `app.sub_title == f"context: {name}"`
+  (shown in the Header). Set on mount from `cli.contexts()` and after every switch.
+  If `contexts()` fails: error notify, sub_title `context: ?`.
+- Key `c` opens `ContextScreen(ModalScreen[str | None])` with `OptionList#contexts`,
+  one option per context, option id = context name, current one marked with `*`.
+  Selecting an option dismisses with its name; `escape` dismisses with `None`.
+- `c` while any guest is busy -> warning notify, no screen (VMIDs collide across
+  clusters, so the busy guard would point at the wrong cluster).
+- Selecting the current context or dismissing -> no CLI call.
+- A reload started before a switch must not refill the table (generation counter);
+  `g` and `c` are refused while a switch runs; the post-action reload finishes before
+  the guest stops being busy.
+- Selecting another context: table cleared immediately (old cluster's guests must not
+  stay actionable), then in a worker `cli.use_context(name)`, sub_title updated, guests
+  reloaded. On `CliError`: error notify with summary, sub_title re-read from
+  `cli.contexts()`, guests reloaded.
+
+## Out of scope
+
+Adding/editing/deleting contexts, `config verify`, restoring the original context on exit.
+
+## Steps
+
+- [x] Tests for `contexts`/`use_context` and the UI contract above - red first.
+- [x] Implementation - full suite green, ruff clean.
+- [x] Live: open the list, switch to `mainframe-edge` and back to `mainframe`
+      (only `config use` and read-only listing; no guest actions); final context
+      is `mainframe`.
+- [ ] Push `feat/guest-tui-mvp`, merge to `main` (user-approved).