fix(core): preserve spawned-agent task payload and profile

[andrei-hasna]

Why

spawn_agent could omit a large encrypted initial task before the child’s first model sample, and a forked child could inherit a stale turn/profile default instead of the parent session’s live selected auth profile.

What

• Deliver the spawned child’s initial task through a stable spawn-call message ID.
• Preserve the initial task as the provider-native model input item, bypassing only the ordinary mailbox size clamp for this one spawn delivery.
• Retain delivered IDs so retries before or after the first drain cannot duplicate the task.
• Refresh the child config from the parent session’s live auth-profile selection before applying supported overrides.
• Keep ordinary mailbox traffic bounded and leave runtime-native send_message behavior unchanged.

Test plan and current evidence

• Failing-first Blacksmith run 31276010499 on tests-only SHA de9d7494b36dd66ed43b083f82791b2bcf3bd493: failed twice at assertion failed: child_request.body_contains_text("agent_message").
• Remediation run 31276361125 on 60c766664eae68c6c79af21cf83d15f977c46576: reproduced the same behavioral failure, showing the first fix lost the native item shape.
• Remediation run 31276892457 on a66da29f0624b1c0f4e8a33cc5026ae23183b4e0: compile failure from one stale unit-test reference; the obsolete test was removed.
• Final focused run 31277205960 is in progress on exact head a723e537f68a546d7e02134f5150646c68093940, covering:
  • verbatim exact-once initial task for fork_turns=none, a small positive count, and fork_turns=all;
  • live selected healthy-profile inheritance;
  • no stale project-default substitution;
  • explicit all-profiles-exhausted failure behavior.
• cargo fmt -- --check: exit 0.
• git diff --check: exit 0.
• Staged security scans: zero findings on the 11-file implementation, 3-file first remediation, and 1-file final remediation.
• Pre-push hook: [pre-push] scanning 1 commit(s) for HEAD.

This PR remains draft pending the terminal focused result, exact-head required checks, and one independent adversarial review. It must not merge or publish before those gates.

Todos: e7ce510d-bbf8-4fc1-8252-6af983d529e0

Agent: Theophrastus