chore: prepare Heddle v4.5.0

[roackb2]

Summary

• prepare @roackb2/heddle and @roackb2/heddle-remote 4.5.0 in lockstep
• document the new typed, sanitized model-failure contract for hosted products
• record the real v4.4.0..HEAD release scope and downstream BYOK validation

Why

Hosted products that accept user-supplied model credentials need to distinguish an invalid credential from generic execution failure without parsing provider messages or leaking provider diagnostics. Heddle #243 now owns that classification through its existing retry, finisher, loop, and conversation-turn path; this release makes the merged contract available to downstream servers from npm.

Validation

• yarn lint
• yarn typecheck
• yarn test — 674 unit and 293 integration tests
• yarn build
• root and remote npm pack --dry-run
• exact local @roackb2/heddle@4.5.0 tarball installed into the SlideX BYOK server
• SlideX server: 37 tests, typecheck, and production build

Release handoff

After this PR is merged, create the annotated v4.5.0 tag and GitHub release from docs/releases/v4.5.0.md, then publish @roackb2/heddle-remote before @roackb2/heddle.

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