feat: add telemetry to all skills

[vahidlazio]

Summary

• Adds agent-telemetry event tracking to all 10 skills that were missing it: 4 migration skills (posthog, eppo, statsig, optimizely), 5 warehouse setup skills (router + bigquery, snowflake, databricks, redshift), and the dry-run skill
• Matches the telemetry pattern already established in onboard-confidence — session ID generation, key acquisition from onboarding.confidence.dev, fire-and-forget event publishing to events.confidence.dev
• Each skill has tailored step and action examples specific to its workflow (e.g. migration skills use plan-flags.scan-source, warehouse skills use bigquery.create-warehouse)

Test plan

• Verify telemetry setup curl acquires a key when running any migration skill
• Verify telemetry events publish in the background without blocking the user flow
• Verify failed telemetry calls are silently ignored
• Verify dry-run skill sends real telemetry despite simulating all other API calls

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