Show native task child model provenance and document model levers

[leoncheng57]

Stacked on the session-agent PR. Narrows #90 to its remaining upstream dependency.

What

Native task child model provenance: the delegated-work panel now shows the model each native child actually ran with, read from the task part's launch metadata. Plus documentation of the model levers that exist today, and the AGENTS.md record of the deployed #75 fix.

/sessions/ses_mock_subagent_parent?directory=/tmp/mock-subagent-project

Why this scope

Upstream's task tool has no per-delegation model parameter (verified in source at v1.18.22 and dev HEAD: child model = subagent's configured model ?? parent's current model). Full per-delegation selection is therefore upstream work; what this app can do honestly is (a) show the model that was actually resolved, and (b) document the two real levers — pin a model on the agent definition, or switch the parent model before delegating. docs/subagents.md gains that section.

• TaskLaunch/SubagentTask carry model (native rows only; Managed Children keep requestedModel), coalesced across resume parts, malformed metadata ignored rather than guessed.
• AGENTS.md decision 19 records the deployed state of Allow Build Task children after a parent previously used Plan #75: upstream fix fix(agent): stop inheriting superseded parent session denies in subagents anomalyco/opencode#45064, local v1.18.22-dca binary under the ai.opencode.serve LaunchAgent, live verification, and the pin-bump path back to stock. docs/subagents.md's fresh-parent workaround is annotated as stock-build-only.

Testing

• Unit: model extraction, resume coalescing, malformed-metadata rejection, derive-to-row carriage.
• e2e: native row model assertion on the existing derived-ledger test.
• Full suite: typecheck, 522 unit, build, 287 e2e.