fix(delegate): inherit parent fallback_chain in _build_child_agent

[nftpoetrist]

Root Cause

_build_child_agent constructed every child AIAgent without passing fallback_model, so _fallback_chain = [] for all subagents.

When a subagent hit a rate-limit or exhausted its credential pool, the runtime fallback check in run_agent.py found an empty chain and failed immediately — even though the parent agent had fallback_providers configured and would have recovered from the same error.

# run_agent.py — runtime fallback check (lines 7486, 12267)
if self._fallback_index >= len(self._fallback_chain):
    # child always lands here: 0 >= 0 → no fallback attempted

Parity Gap

The cron scheduler already propagates fallback_model correctly:

# cron/scheduler.py:1038
agent = AIAgent(..., fallback_model=fallback_model, ...)

_build_child_agent was missing the equivalent pass-through.

Fix

Read the parent's _fallback_chain (the normalised list form that AIAgent.__init__ already accepts via its fallback_model parameter) and pass it to the child:

parent_fallback = getattr(parent_agent, "_fallback_chain", None) or None

child = AIAgent(
    ...
    fallback_model=parent_fallback,
    ...
)

Empty chains coerce to None so AIAgent initialises _fallback_chain = [] as usual rather than re-filtering an empty list.

Test Plan

• test_child_inherits_fallback_chain — verifies non-empty _fallback_chain is passed as fallback_model
• test_child_gets_no_fallback_when_parent_chain_empty — verifies empty chain produces fallback_model=None
• Both tests pass (2 passed)

[alt-glitch]

Related to #15285 (closed, same fix). Verify #15285 was rejected vs just abandoned before merging this.

[alt-glitch]

Related to closed #15285.

[teknium1]

Salvaged via #19601 onto current main — the delegate fix commit was cherry-picked with your authorship preserved. The Slack setup commit from the same branch was salvaged separately as #19583. Thanks @nftpoetrist!