Delegate trace commands to plugin runtime

[Stephen Belanger (Qard)]

Summary

• pin the plugin runtime from braintrust-coding-agent-plugins#20
• mount the exported bt_daemon::TraceArgs tree under the top-level bt trace namespace
• delete src/agents.rs and delegate the complete trace command to bt_daemon::run_trace
• retain only a small host adapter for Braintrust credential leases and destination selection

Ownership boundary

bt no longer matches or interprets any coding-agent trace subcommand. It hands the parsed trace command to the plugin runtime with the current executable mount prefix and an implementation of the host-service interface.

The plugin repository owns setup, daemon lifecycle, hooks, status, stop, imports, managed runs, output rendering, agent package identities, marketplace behavior, and settings reconciliation. bt continues to own its profile store, OAuth/API-key handling, project picker, and top-level Clap mount.

This PR depends on braintrustdata/braintrust-coding-agent-plugins#20 and pins its exact commit.

Validation

• cargo fmt --all --check
• cargo check --locked
• cargo test --locked --test cli trace_ (13 passed)
• cargo test --locked --quiet -- --test-threads=1 (803 passed)
• cargo clippy --locked --all-targets --all-features -- -D warnings -A clippy::collapsible_match
• git diff --check

Strict Clippy without the targeted allowance remains blocked by 12 existing collapsible_match findings in unrelated files.

[github-actions]

Latest downloadable build artifacts for this PR commit bf03da0a31e6:

• Workflow run: https://github.com/braintrustdata/bt/actions/runs/31640917651
• Download all artifacts (GitHub CLI): gh run download 31640917651 --repo braintrustdata/bt
• Installers are published from main automatically. To publish one for a PR branch, run release-canary manually via workflow_dispatch.

Available artifact names

• artifacts-build-global
• artifacts-build-local-aarch64-pc-windows-msvc
• artifacts-build-local-x86_64-unknown-linux-musl
• artifacts-build-local-x86_64-unknown-linux-gnu
• artifacts-build-local-x86_64-pc-windows-msvc
• artifacts-build-local-x86_64-apple-darwin
• artifacts-build-local-aarch64-apple-darwin
• artifacts-build-local-aarch64-unknown-linux-gnu
• artifacts-plan-dist-manifest
• cargo-dist-cache