feat(auto-engineer): make plan/implement turn and budget limits configurable per run

Problem: The auto-engineer's per-phase turn and budget caps are hardcoded — config/defaults.yml (max_plan_turns: 20, max_plan_budget_usd: 1.50, etc.) with defaults baked into scripts/run-claude.sh (MAX_CLAUDE_TURNS, MAX_BUDGET_USD). They aren't exposed on the workflow_dispatch, so there's no way to raise them for a single run from the Actions UI.

A plan run on #99 hit the cap and exited with code 1 and 0 lines of output ("likely hit max-turns or budget"), with no self-serve way to give it more room — see run https://github.com/mozilla/blender/actions/runs/30034409270. This was the main thing blocking us from using the auto-engineer.

Proposal: Add optional workflow_dispatch inputs — e.g. max_turns and max_budget_usd — that override the per-phase defaults, plumbed through to MAX_CLAUDE_TURNS / MAX_BUDGET_USD in run-claude.sh. Empty/unset falls back to config/defaults.yml (current behavior).

Acceptance: Can bump the plan/implement turn and budget limits for a single dispatch from the Run-workflow form without editing the repo.

Start with the workflow that defines workflow_dispatch, then trace its inputs into config/defaults.yml and scripts/run-claude.sh. Verify how MAX_CLAUDE_TURNS and MAX_BUDGET_USD are currently populated and test a dispatch with empty and overridden values. Done means the Run workflow form can change plan and implement limits for one run while empty inputs preserve current defaults.