Add Tabstack API catalog integration

[snowmead]

Summary

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

Test plan

• cargo nextest run -p stackless-integrations -E 'test(tabstack)'
• 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 auth, existing providers, or shared provisioning paths beyond a new registry row.

Overview
Adds Tabstack as a managed Stripe Projects catalog integration for tabstack/api, so stacks can provision Tabstack and wire api_key into service env (e.g. ${integrations.res.api_key}).

New providers/tabstack module defines TabstackApi: global-only config with optional plan (trial/individual/team/pro), resource kind integration-tabstack, and provisional OUTPUT_FIELDS until discover pins them. validate_config is a no-op stub, matching several similar API providers.

Wires the provider into register_providers! and extends catalog_outputs_match_output_fields so Hostable::OUTPUTS stays aligned with catalog output names.

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