fix(subagent): return the chain's final result and mark unstarted steps

[Uking-xxx]

Fixes #220

Problem

A blocking subagent chain returned records[0] as the tool content (execute.ts), sharing the single-mode path. The parent model only sees content, never details, so:

• a successful chain handed the parent the first step's output, not the final one;
• a chain that failed or was aborted mid-way still read as the first step's success — the failure only lived in details.results;
• steps that never started were seeded as running and stayed running after the chain stopped (in the TUI too: a ticking timer and (running...)).

Partial updates also flipped details.mode from chain to single mid-run.

Changes

• Chain content: a per-step status line, plus
  • on success: the last step's output;
  • on failure/abort: Chain stopped at step N/M (...); the chain did not complete., the stopping step's error, and the last completed output (truncated at 50k).
    Earlier step bodies stay in details.results rather than piling into the parent's context.
• New record statuses:
  • queued — not dispatched yet (later chain steps, parallel tasks behind maxConcurrency); no startedAt, so no clock. Flipped to running when the step is dispatched.
  • skipped — chain steps after a failed/aborted step.
• Partial updates keep mode: "chain".
• TUI: icons/labels for queued/skipped; widget and parallel headings count them separately (skipped no longer counted as failed); chain heading reads N/N steps completed / failed at step 2/3 instead of results[0].status.
• Background lanes treat queued records as still running.

Single and parallel content are unchanged.

Testing

• New test/subagent-chain-result.test.ts (8 cases, deterministic runner): success, mid-chain failure, abort, first-step failure, queued→running ordering for chain and parallel, plus single/parallel unchanged. 4 of them fail on main.
• All subagent tests pass (52/52); pnpm run check passes.
• Manual tmux run from source against step-5-preview: successful chain, unknown-agent failure at step 2, and Esc during step 2 — parent got the expected content in each case, and the widget showed 1/3 complete, 1 running, 1 queued while step 2 ran.

Note for consumers

StepSubagentResultRecord.status gains queued and skipped. Anything rendering details.results[].status directly (e.g. Desktop) should handle both.