fix(hook-kit): repair agent-spawn model injection and gate high-tier spawns

[DrumRobot]

Two fixes to block-agent-spawn-without-model.sh, split so the bug fix is cherry-pickable independently of the new policy gate.

Changes

fix(hook-kit) — preserve tool input when injecting the default model

Gate A emitted updatedInput: { model: "sonnet" }, but updatedInput REPLACES the tool input wholesale rather than being merged into it. Every spawn without an explicit model therefore lost description / prompt / subagent_type and died on schema validation with "The required parameter description is missing".

The gate was meant to make model-less spawns cheap, not impossible — it turned them into a hard failure, and the only workaround was to always pass model explicitly, which defeats the default. Now echoes the caller's own tool_input back with just model added.

fix(hook-kit) — gate opus/fable spawns on explicit user approval

Gate A only fires when model is absent, so it said nothing about a spawn that explicitly asks for an expensive tier. Choosing WHAT work to run and approving WHICH TIER runs it are separate axes; collapsing them is what produced an unapproved high-tier spawn after a work item was selected and the selection was read as tier consent.

Gate C blocks opus / fable unless the prompt carries [tier-approved], or an AskUserQuestion tool_use appears in the current turn. The turn window is computed the same way edit-guard.sh does it, and an AskUserQuestion answer arrives as a tool_result rather than a user string, so the ask stays inside the window. sonnet / haiku spawns and the Gate A default path are unaffected.

Test plan

• Model-less spawn now yields the full input plus model: sonnet (was: schema-validation failure)
• Explicit model still emits nothing (no injection)
• Gate B unchanged — missing run_in_background still exits 2
• Gate C: opus and fable without approval exit 2; [tier-approved] passes; an in-turn ask passes; sonnet unaffected
• bash -n clean; full pre-push suite green (24 tests) on push