Correlate per-child subagent progress and preserve pipeline accounting

[rochecompaan]

Parent: #116

Depends on #123 and #124.

Outcome

Patchmill correlates authoritative parent-session entries to the requested subagent children, preserves one observation opportunity per child, and leaves existing parent-level accounting and pipeline progression intact.

Scope

• Parse and validate patchmill-subagent-progress custom entries from the exact parent session.
• Inventory child requests using the adopted pi-subagents execution-shape contract:
  • simultaneous nonempty chain and tasks is invalid;
  • exactly one aggregate mode ignores top-level agent;
  • otherwise use direct mode.
• Correlate by parent tool-call ID and child index.
• Suppress exact duplicate metadata tuples while emitting an additional observation for every changed authoritative tuple without replacing earlier output.
• Emit exactly one unresolved fallback for every child that receives no authoritative tuple.
• Preserve independent visibility for failed and unresolved children.
• Count one parent toolCalls accounting unit per parent toolCallId, regardless of child update count.
• Preserve implementation todo refresh from subagent progress.
• Preserve async and terminal fallback cardinality.

Fallback data boundary

Synthetic unresolved-child fallbacks are task-free, agent-only progress observations. They remain outside LLM context and contain no task text, child output, prompts, credentials, full result objects, or unrestricted result metadata.

Out of scope

• Pi lifecycle hook implementation.
• Session-file ownership and command cancellation.
• Console line formatting.
• Enriching triage output.

Acceptance criteria

• Direct, counted, parallel, and chain requests produce the expected child inventory.
• Conflicting aggregate modes do not produce a guessed inventory.
• Exact duplicate tuples are suppressed per parent call and child while each changed authoritative tuple emits an additional observation.
• Every child without an authoritative tuple receives exactly one task-free, agent-only fallback outside LLM context.
• Parent toolCalls accounting remains one unit per parent tool call.
• Implementation todos can advance from child progress before the parent tool finishes.
• Production runPiPrompt() wiring is covered, not only helper-level fixtures.

Planning constraints

This issue owns one focused correlation/accounting PR. Its spec and plan supersede only the child-inventory, session-entry parsing, gating, fallback, accounting, and pipeline-progression portions of the deprecated monolithic plan referenced by #116.