Add Steel browser catalog integration

[snowmead]

Summary

• Phase 1 CatalogResource for steel/browser (Wave 7).
• Registry row + providers/steel module, gap + hermetic provision tests.
• Provisional OUTPUT_FIELDS (api_key) until mise run discover steel/browser.
• Branched from refresh tip 30df3de (plugin v0.25.0 fixtures). No smoke fixture.

Test plan

• cargo nextest run -p stackless-integrations -E 'test(steel)'
• registry uniqueness + catalog_outputs_match_output_fields
• mise run check (pre-push hook)

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Note

Low Risk
Additive provider registration and tests only; no changes to shared provisioning or auth paths beyond a new catalog row.

Overview
Adds Steel as a managed Stripe Projects integration for the steel/browser catalog service so stacks can provision it and wire api_key into service env (e.g. ${integrations.*.api_key}).

New providers/steel module with SteelBrowser: global-only config (optional name), resource kind integration-steel, and provisional OUTPUT_FIELDS until mise run discover steel/browser. SteelBrowser is registered in the provider registry and included in the catalog_outputs_match_output_fields hygiene test.

Tests cover catalog schema alignment against the shared fixture and a hermetic provision path that asserts api_key is recorded on the resource payload.

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