What

• Add optional summary to ExecCommandEndEvent with cwd/stdout_tail/stderr_tail and truncation flags.
• Generate the summary once in core for non‑zero exit codes; clients remain backwards‑compatible.

Why

• Reduce time lost to opaque failures by surfacing actionable, compact details (without dumping full streams into logs).

How

• Protocol extension (BC): summary: Option<ExecCommandSummary>.
• Core: when exit_code != 0, compute 1KB tails from stdout/stderr and include truncation info.
• TUI/exec: pattern‑match with .. to ignore new field; no behavior change required.

Before/After

• Before: ExecCommandEnd contained full truncated streams only; errors in logs were often unhelpful.
• After: on failures, summary captures cwd + tails so logs and users see the key failure context quickly.

Tests

• Full suite passes (cargo test, cargo clippy --tests, cargo fmt).
• Optional serde unit test can assert that both Some(summary) and None serialize/deserialize.

Risks

• Slightly larger event payloads on failures (bounded tails). Secrets masking is handled by existing log policy.

DCO/CLA

• Signed‑off‑by present on commits. Will sign CLA via comment on this PR.