feat(init): support existing task folder via --tasks-root

[HenryLach]

Summary

Add a first-class taskplane init flag for projects that already have a task folder.

Changes

• add --tasks-root <relative-path> init option
• validate that --tasks-root is relative and stays within project root
• default to skipping example task packets when --tasks-root is provided
• add --include-examples to opt back into example scaffolding with custom root
• update CLI help and docs (README, install tutorial, commands reference)
• update changelog Unreleased

Why

This supports existing setups like docs/task-management without creating an alternate taskplane-tasks/ area by default.

Validation

• node bin/taskplane.mjs help
• node bin/taskplane.mjs init --preset full --tasks-root docs/task-management --dry-run --force
• node bin/taskplane.mjs init --preset full --tasks-root docs/task-management --include-examples --dry-run --force
• absolute path rejection + conflicting flags rejection
• markdown relative link check