fix(minimal): task box review nits and test gaps from #2999

Residual of BOX T43 (#2405, PR #2999): the smaller findings from the pre-merge review, collected in one issue. Split any of them out if it grows.

Behaviour:

A PTY-enabled task launches its entrypoint before it is refused (crates/minimald/src/exec/process.rs, about line 857). Refuse before the launch.
A Ctrl-C after yield_to_handler can turn an exit 0 into 130 (crates/minimal/src/task.rs). Disarm the interrupt guard once the run has its exit code.
The kept-box line prints on the Ok(None) path, where the box may still run. It must say the box may still be running and point at min box show <box>.
The interrupt path never prints the kept-box line.
A first run whose client leaves while the task prints nothing keeps its stream receivers open until the next chunk, so a rejoin is refused until then (follows from the D1 fix).

Code quality:

Blocking std::fs::read runs in async code (crates/minimald/src/exec/process.rs, about line 768). Use tokio::fs or spawn_blocking.
A task variable named PATH reaches the entrypoint through command.env and skips the $PATH expansion that sandbox::Config::command_env gives a composed PATH (arch note on 0b31489).
"task box ended" naming in log and code.
Mixed ... and … in printed lines.
clippy-strict hits on lines from T43 — Run a task as a task box that returns its exit code #2999: task.rs (let_underscore_must_use ×2, assertions_on_result_states), pipes.rs drain (indexing_slicing ×2), process.rs (let_underscore_must_use ×2), box_actor/tests.rs (let_underscore_must_use).

Test gaps:

task_run_exits_with_held_open_stdin_pipe now covers only the refusal path. Restore coverage of the held-open stdin case.
entrypoint_started has no unit test.
The e2e interrupt step uses a fixed sleep 3. Wait on a marker instead.

A cancelled run can leave its stage behind. The real fix is #3043.

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