feat: deep-agent canvas, live observability, and multi-environment tooling

Self-hosted platform for building, testing, and shipping LangChain/LangGraph agents. Deep-agent sub-agents on the canvas, a live tracing/observability timeline, auto-provisioned built-in tools with import/export, per-environment tool variables, streamed evaluations, and per-user auth token forwarding.

.github/PULL_REQUEST_TEMPLATE.md

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+<!-- Thanks for contributing to Forge! Please fill this out to speed up review. -->
+
+## What & why
+
+<!-- What does this change do, and what problem does it solve? Link the issue: Closes #123 -->
+
+## Type of change
+
+- [ ] Bug fix
+- [ ] New feature
+- [ ] Performance
+- [ ] Refactor (no behavior change)
+- [ ] Docs / chore
+
+## Checklist
+
+- [ ] Backend: `ruff check forge migrations` is clean and `pytest -q` passes (from `apps/api`)
+- [ ] New/changed backend code is `mypy`-clean
+- [ ] Frontend: `pnpm --filter web build` passes (from repo root)
+- [ ] Shared schemas (`packages/schemas`) updated if node/tool config changed
+- [ ] Tests added/updated for the change (characterization test for behavior-preserving refactors)
+- [ ] `CHANGELOG.md` updated under **Unreleased** (for user-facing changes)
+- [ ] No secrets, tokens, or customer data in the diff
+
+## Notes for reviewers
+
+<!-- Anything that needs special attention, screenshots for UI changes, migration notes, etc. -->