fix(sdk): verify agent ownership in eval routes

[manpreet171]

Problem

validateConnectKey() returns a service role Supabase client, which bypasses RLS. Routes that accept an agent_id from the request therefore have to check ownership themselves — traces, command, checkpoint and execution all do this.

The four evals routes don't. They authenticate the caller and then query using ids taken straight from the request body:

Route	Effect with any valid connect key
GET /api/sdk/evals/regression	read another user's regressions via agent_id
PATCH /api/sdk/evals/regression	acknowledge another user's regression via regression_id
POST /api/sdk/evals/regression	read their eval results and insert rows
POST /api/sdk/evals/results	write results / create eval sets under their agent
POST /api/sdk/evals/from-trace	read their checkpoints, tool executions and reasoning steps via task_id

The GET and PATCH cases need no plan gate, so a free account is enough.

Fix

Added an ownsAgent() helper in lib/sdk-auth.ts and used it in the routes that were missing the check. Where a route accepts an eval_set_id, regression_id or task_id, that id is resolved back to its agent (or user) first, so a caller can't pair their own agent_id with someone else's record.

Responses follow the existing convention: 403 with Unauthorized agent access.

Tests

tests/evals_security.test.ts mirrors the structure of tests/traces_security.test.ts. 6 of the 8 new tests fail on main and pass with this change.

Full suite: 174 passed, and tsc --noEmit is clean.

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