Support OpenCode and Pi trace setup

[Stephen Belanger (Qard)]

Summary

• add bt trace setup opencode and bt trace setup pi
• configure OpenCode's published npm plugin and install Pi's published npm extension
• write profile, organization, and project selection to each agent's own braintrust.json
• keep managed OpenCode and Pi runs isolated from their persistent configuration
• pin the coordinated plugin-monorepo implementation at 952e2f26c8613d3fac82ca4999aae448a0b82139

Validation

• full cargo test --locked (715 unit tests plus all integration suites)
• focused setup and managed-run CLI tests
• cargo fmt --all -- --check
• cargo check --locked
• git diff --check

Strict clippy currently reaches only unrelated pre-existing collapsible_match failures under Rust 1.95 in datasets, SQL, and traces code; this PR does not modify those files.

Depends on the stacked plugin-monorepo PR.

[github-actions]

Latest downloadable build artifacts for this PR commit 390e813dce89:

• Workflow run: https://github.com/braintrustdata/bt/actions/runs/31418876100
• Download all artifacts (GitHub CLI): gh run download 31418876100 --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-x86_64-apple-darwin
• artifacts-build-local-aarch64-pc-windows-msvc
• artifacts-build-local-x86_64-pc-windows-msvc
• artifacts-build-local-aarch64-apple-darwin
• artifacts-build-local-x86_64-unknown-linux-musl
• artifacts-build-local-x86_64-unknown-linux-gnu
• artifacts-build-local-aarch64-unknown-linux-gnu
• artifacts-plan-dist-manifest
• cargo-dist-cache