Fix stale increment_sessions_used RPC example in contributor instructions

[Taleef7]

Context

The current database function signature is increment_sessions_used(p_request_id), but .github/copilot-instructions.md still shows an example using p_package_id.

Why this matters

Future agents/contributors may copy the stale example and implement code against a non-existent RPC parameter.

Acceptance criteria

• .github/copilot-instructions.md uses p_request_id in the RPC example.
• Any other repo docs or agent instructions with the stale p_package_id example are corrected.
• A quick rg "p_package_id|increment_sessions_used" pass confirms no misleading examples remain.

References

• Over-closed audit finding: .github/copilot-instructions.md:104-108
• Related current schema: supabase/migrations/20260225000002_create_sessions_table.sql, later hardened migrations