fix: unify user_agent to "clawd-rust-tools/0.1"

[LLuke]

Summary

• adding missing and unify user_agent

Anti-slop triage

• Classification: actionable-fix
• Evidence: Missing user agent in request to custom openai compatible providers #3060
• Non-destructive review result: merge candidate

Verification

• Targeted tests/docs checks ran, or the gap is explicitly recorded.
• git diff --check passes.
• No live secrets, tokens, private logs, or unrelated generated churn are included.

Resolution gate

• If this PR resolves an issue, the issue number and fix evidence are linked.
• If this PR should not merge, the rejection/defer rationale is evidence-backed and does not rely on vibes.
• I did not merge/close remote PRs or issues from an automation lane without owner approval.