Support configurable forge-loop base branch

[hadamrd]

Summary

Adds a configurable repository base branch so forge-loop can operate on repos whose default integration branch is not trunk.

Why

Pulsar uses main. The runner was hardcoded to fetch and create worktrees from origin/trunk, so every worker failed before doing useful work with fatal invalid reference origin/trunk. This should be a manifest/env setting, not a runner constant.

Changes

• Add repo.base_branch to config, defaulting to trunk for existing users.
• Add LOOP_BASE_BRANCH env override.
• Thread the setting through worker dispatch, pipeline dispatch, boot dispatch, MCP worker dispatch, and doctor/config output.
• Update worker worktree preparation to fetch and base from origin/<base_branch>.
• Add config tests for YAML and env override.

Verification

• uv run --frozen pytest tests/test_config.py tests/test_worker.py
• Pulsar wrapper now reports code matches origin/main and can create loop worktrees from origin/main.