Add safe worktree bootstrap

[palewire]

Summary

Add an idempotent make bootstrap workflow for primary checkouts and linked Git worktrees. The typed Python bootstrap discovers checkout paths from Git metadata, safely shares an ignored .env without overwriting local files, maintains a stable WORKTREE_ID in an ignored .env.worktree, preserves local overrides, and runs the locked uv dependency sync.

Expand contributor and agent guidance with dotenv precedence, worktree isolation, shared-service safety, conflict-hotspot coordination, generated-file rules, and the limits of pytest-xdist resource isolation.

Acceptance Criteria

• The behavior matches the issue or request.
• Relevant edge cases are covered.

Validation

• make bootstrap UV=$HOME/.local/bin/uv PYTHON=.venv/bin/python
• make verify UV=$HOME/.local/bin/uv
• Pre-commit and pre-push hooks

Documentation and Release Notes

• Documentation is updated, or no update is needed.
• CHANGELOG.md is updated for a user-facing, compatibility, or security change.