Spawn: close pre-existing child-runtime semantic gaps

[grzegorznowak]

Context

Follow-up from PR #16 (Pi 0.80.8 spawn runtime compatibility). These behaviors predate that compatibility repair and should be addressed separately from its renderer, thinking-reporting, and compatibility-script fixes.

Scope

Executable tool inheritance

• Define which parent tool sources child spawn supports: rediscovered project/user/package extensions, SDK customTools, inline/dynamic extension factories, CLI/additional extension paths, and custom resource-loader state.
• Replace or constrain name-only inheritance so a documented inherited tool is present in the actual child registry with the intended implementation.
• Prevent same-name rediscovery from silently substituting different child semantics.
• Explicitly omit or diagnose unsupported source channels.
• Update README wording to distinguish allowed names, actual registry entries, and semantic inheritance.
• Add real-child integration tests for SDK-only, inline/dynamic, additional-path, and same-name collision cases.

Cancellation reason preservation

• When signal cancellation occurs and prompt() resolves without assistant text, return an aborted outcome with the supplied reason before applying the generic no-output fallback.
• Add regressions for cancellation with no text, cancellation after partial text, genuinely empty non-cancelled output, and existing stale/error precedence.

Acceptance criteria

• Every tool documented as inherited executes the intended parent-visible implementation in a real child session.
• Unsupported tool sources are explicitly excluded or diagnosed rather than silently allowlisted.
• Parent overrides cannot silently become same-named builtins or rediscovered extensions in the child.
• Empty-output signal cancellation preserves outcome: "aborted" and the cancellation reason.
• Non-cancelled empty output and stale/error precedence retain their current behavior.

Non-goals

• Serializing arbitrary closures or extension state without an explicit support decision.
• Redesigning cleanup-error observability.
• Handling hypothetical future rejection from Pi's currently non-rejecting AgentSession.abort().
• PR Pi 0.80.8 spawn runtime compatibility #16 renderer recovery, effective-thinking reporting, compatibility-script portability, or CI matrix hardening.