Browser-agent fallback for gated platform APIs

[gidutz]

Why

Several high-value ad-platform capabilities are gated behind program approvals or limited OAuth scopes that take days–weeks to obtain — and a self-hosted YieldAgent user may never get them. Concrete examples already in the codebase:

• LinkedIn Lead Sync (r_marketing_leadgen_automation) — separate program approval; required for pull_lead_responses / subscribe_lead_webhook.
• LinkedIn Matched Audiences (rw_dmp_segments) — separate Audiences-program approval; required for create_dmp_segment, upload_audience_csv, add_dmp_users.
• LinkedIn Advertising API itself is a Vetted Product — even creating campaigns requires partner-tier access.
• Google Ads Developer Token — starts at Test access (test accounts only); Basic/Standard needs application.
• Meta Marketing API App Review — dev-mode apps can only authenticate Admins/Devs/Testers.

When scope is missing, the agent should not silently fail. It should fall back to driving the platform's web UI through a browser — Playwright session via the existing mcp-browser-control tool registry entry (browser.run_playwright_flow, risk: credential_sensitive, approval: required).

Proposed pattern

Every MCP tool that wraps a gated API surface gets a paired `*_via_browser` fallback, OR the existing tool detects 403 / scope errors and emits a structured `needs_browser_fallback` response the agent can route on. The agent's tool-selection logic should:

1. Try the official MCP tool first (cheaper, reversible, no UI breakage risk).
2. On 403 `USER_NOT_AUTHORIZED` / unmapped-scope error, surface the option to retry via browser.
3. Browser path always goes through the approval gate (`credential_sensitive` risk) — never auto-triggered.

Where this matters next

• LinkedIn Lead Sync pull → if the org hasn't been approved for `r_marketing_leadgen_automation`, scrape Campaign Manager's Lead Center.
• LinkedIn Matched Audiences upload → if `rw_dmp_segments` is missing, drive the Audiences UI to attach the CSV.
• Google Ads Developer Token at Test level → fall back to driving Google Ads UI for any production-account action.
• Meta App Review pending → drive Meta Ads Manager UI for any non-allowlisted advertiser.

Acceptance

• A single rule documented in CLAUDE.md / agent system prompt: "When a platform API returns a scope/approval error, propose the browser fallback via the existing browser-control tool rather than returning the error to the user."
• Each MCP tool that touches a gated surface either auto-retries through the browser path or returns a structured `{ needs_browser_fallback: true, reason, fallback_tool }` payload.
• The browser fallback always emits a dry-run preview first and routes through the approval gate.