Strip the sub-agent report envelope from expanded task details

[TheGreatAxios]

Summary

Stacked on #372 (CL-4869) — both tickets concern how sub-agent
activity renders in the parent transcript.

Auditing the live TUI's task/tool-row rendering against CL-4866's
outcomes:

• Curated task rows, not raw field dumps: already correct.
  describeToolCall's "task" case (src/tui/tool-formatter.ts)
  reduces a spawn's prompt/intent/tier/maxTurns/criteria brief
  to just its description, and toolCallRow uses it. Confirmed with
  a rendered frame below.
• Long shell commands hang-indented, not wrapped mid-token: already
  correct and tested (command-display.test.ts, stream.ts
truncateLine/indentBody).
• Repeated calls collapse onto one row: already correct and tested
  (tool-rows.ts canCoalesceCall/coalesceCallRows, "a run of
  identical calls / is one row").
• Sub-agent report markdown rendered or stripped: not correct.
  summarizeTaskResultPreview — which strips the Sub-agent "desc" reported: / ## Summary envelope — existed in
  src/tui/tool-formatter.ts with zero callers anywhere in the
  codebase. The one-line collapsed preview happened to go through it
  indirectly via summarizeToolResult, but the expanded detail behind
  the arrow fell through to bodyLines(content) on the raw text, so
  expanding a task result showed the literal Sub-agent "..." reported:
  prefix and ## heading markers.

This PR fixes the one real gap: resultSummary in mcp-view.ts now
gives "task" results the same always-curated treatment tool_search
catalogues already get, and strips the envelope/heading markers from
the text that becomes the expanded detail.

Investigation note (CL-4869 connection)

Digging into why these fixes seemed to exist already (PR #298/#300,
superseded by merged #312) turned up something more important: #312
touched only src/tui/app.tsx and its Ink component tree, which has
zero importers anywhere in the repo — package.json carries no
ink/react dependency, and the live entrypoint renders through
src/tui-opentui/shell.ts instead. Those merged fixes never reached
the shipping TUI. That's documented in #372's description. This PR's
stripTaskReportEnvelope addition is new, not a port of dead-tree
code — the dead tree's equivalent (mergedToolCollapsedPreview) was
unused there too.

Verification

• bun run typecheck
• bun run test (full suite, 4040 pass / 0 fail)
• bun test ./src ./tests ./evals --randomize --seed 42 (4040 pass / 0 fail; one earlier run on this seed showed an unrelated one-off flake in an unrelated file that did not reproduce on rerun)
• Rendered a real frame of an expanded task result row through the
  OpenTUI test harness:

  ✓ Task explore callers 8 lines ▾
      Summary
      Found 3 call sites in src/tui-opentui.

      Findings
      - shell.ts line 40
      - diff.ts line 12

No raw prompt/intent/tier dump on the collapsed line, no
Sub-agent "..." reported: prefix, no literal ## in the expanded
body.

Closes CL-4866

[linear-code]

CL-4866