fix(exec): propagate skips transitively under keep-going; UTF8-safe display_name; reap ephemeral images; fix cache clean

[markovejnovic]

Summary

Four execution/cleanup correctness fixes for the local (hm run) backend:

• exec-skip-propagation-1 (high) — under -k/--keep-going, a failed step's transitive dependents (grandchildren) ran anyway on a clean base image. A skipped step returns exit_code 0, but the downstream gate keyed on exit_code != 0, so a skipped predecessor never tripped it. Adds an explicit failed_or_skipped flag to StepOutcome (set on fail, timeout, cancel, and skip) and gates descendants on that instead, so a skip cascades through BuildsIn chains.
• exec-cmd-utf8-panic-1 (medium) — display_name truncation byte-sliced the command at offset 39 (&cmd[..39]), panicking when byte 39 was not a UTF-8 char boundary. Now truncates by chars.
• vm-ephemeral-image-leak-1 (medium) — uncached leaf steps commit ephemeral:<cid> images that the registry never tracks and nothing ever removed. The scheduler now collects every ephemeral committed snapshot and reaps it at run end (success or failure) via a new StepRunner::reap_snapshots -> HmVm::remove_snapshot path.
• cache-clean-wrong-remediation-1 (medium) — hm cache clean deleted registry.db but left every tagged image, and its docker image prune advice (dangling-only) could not remove them. It now enumerates the registry's snapshot IDs (new ImageRegistry::all_snapshot_ids) and removes each image via the Docker backend before deleting the DB; the misleading prune warning is dropped.

Findings addressed

• exec-skip-propagation-1
• exec-cmd-utf8-panic-1
• vm-ephemeral-image-leak-1
• cache-clean-wrong-remediation-1

Verification evidence

Targeted checks run inside the worktree (touched crates only; full Docker-dependent e2e tests are #[ignore]d and cannot run in the sandbox without starting a Docker daemon):

$ cargo check -p hm-vm -p hm-exec -p harmont-cli
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 1.10s

$ cargo clippy -p hm-vm -p hm-exec -p harmont-cli --all-targets -- -D warnings
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.17s   # no warnings

$ cargo test -p hm-vm --lib
    test result: ok. 11 passed; 0 failed; 0 ignored
    # includes new registry::tests::all_snapshot_ids_returns_every_entry

$ cargo test -p hm-exec --lib
    test result: ok. 24 passed; 0 failed; 0 ignored

The skip-propagation regression guard (keep_going_skips_transitive_dependents, an A->B->C chain asserting B and C are both skipped — no step_start — when A fails) and the ephemeral-image reaping check are #[ignore = "requires Docker daemon"] integration tests in crates/hm/tests/keep_going.rs; they pass per the implementer's verification but are not runnable here under the shared-machine guardrails.

Judge verdict

Approved. No blocking issues. Implementer verification passed.

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Generated by the Harmont release-readiness workflow (automated; needs human review before merge). 🤖