Rename `report_result` to `message_parent`

[monneyboi]

Proposal

Rename the child-side tool report_result to message_parent.

Rationale

• The current name frames the tool as a one-shot final report. In practice the instructions already tell the child to "report" in response to every parent message (revisions, verdicts, blockers), so "reporting to the parent" still reads naturally under the new name.
• The new name better reflects what the channel is: the only way to reach the parent. It also opens the door to richer bidirectional communication later (e.g. progress messages, questions mid-task) without another rename.

Affected locations

• index.ts
  • Tool registration (name: "report_result", label, description) around line 740
  • Child prompt text referencing the tool (lines ~655–700): spawn-time guidance and the injected message_agent follow-up prompt
  • Error strings ("retry report_result with the same result")
• README.md — child tool documentation (~line 25)
• AGENTS.md — extension overview (~line 5)

Notes

• Consider whether the parameter name result should become message (or similar) for consistency.
• Reports-are-outcomes-not-progress guidance should be preserved verbatim; the rename should not loosen delivery semantics (retry with same payload, no double-delivery).