fix(deps): update openai agents sdk to ^0.15.0

[dawsontoth]

Supersedes Renovate #150. Bumps @openai/agents and @openai/agents-extensions from ^0.14.0 to ^0.15.0, adapts getModel to the SDK's new Model typing, and adds a CI Build job so PRs are gated on a clean type-check — which is what surfaced the upgrade's type break in the first place.

For the human reviewer

1. getModel now returns core's Model and casts the aisdk() result once. 0.15.0 adds an optional rawUsage to ModelResponse; the AiSdkModel wrapper declares its getResponse rawUsage as Record<string, unknown> | undefined, which isn't assignable to core's rawUsage?: Record<string, unknown> under our exactOptionalPropertyTypes. AiSdkModel genuinely implements Model, so this is an upstream declaration quirk, not a runtime issue — I bridged it with a single cast at the getModel boundary (and split provider selection into getLanguageModel so there's exactly one cast) rather than sprinkling casts at the two call sites (AgentManager, compactConversation). Reversible; the alternative is per-call-site casts.
2. Build gate added to verify-pr.yaml only, not release.yaml. The release workflow already builds via prepublishOnly before npm publish, so a broken build fails the release regardless; the actual gap was PR-time validation. Say the word if you'd also like an explicit build step in release.

Verification

Rebased onto current main, so the 0.15.0 agents bump is validated alongside the recently landed ai v7 / @ai-sdk/* v4 majors (only package.json + the lockfile conflicted; kept main's ai ^7.0.0). Ran the full local gate against Node 24:

• npm run build → tsup ESM + --dts type-check succeed (fails on main+bump without the getModel change — this is the break the new CI job catches).
• npm run lint (oxlint) → clean.
• npm run format (dprint) → clean.
• npm test (vitest) → 53 files, 345 tests passed.

Review coverage

Authored by Claude (Opus 4.8). No outside cross-model review has run yet — flagging per Harper guidelines step 10; happy to run the cross-model-review pre-push pass before this leaves draft.

[gemini-code-assist]

Code Review

This pull request upgrades the @openai/agents and @openai/agents-extensions dependencies to version ^0.15.0. To accommodate this upgrade and resolve type-declaration mismatches under exactOptionalPropertyTypes, lifecycle/getModel.ts has been refactored. Specifically, the getModel function now returns the core Model type, and its provider-specific logic has been extracted into a new helper function getLanguageModel. There are no review comments, so no further feedback is provided.