feat: add milestone tools for managing epics and milestones (#64)

* feat: add epic tools for managing epics

Add MCP tools for epic management:
- list_epics: List all epics in a project
- create_epic: Create a new epic
- update_epic: Update an existing epic
- retrieve_epic: Retrieve a specific epic by ID

Also includes:
- Integration tests for epic operations
- mise.toml for uv version pinning

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Add deleting an epic

Update integration tests

Address PR review feedback for epic tools

- Fix docstrings to only list actual function parameters
- Add validation for epic work item type (raise ValueError if not found)
- Add priority validation with actionable error messages
- Fix update_epic return type from WorkItem to Epic
- Fix grammar: "a epic" -> "an epic"
- Refactor _get_epic_work_item_type_id to accept client/workspace params
- Remove unused WorkItem import
- Fix test to handle list_epics returning flat list (not paginated)
- Remove extraneous f-string prefixes in test prints
- Add delete_epic to EXPECTED_TOOLS

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Fix epics

* feat: add milestone tools for managing milestones

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* Use latest python sdk

* Use proper plane-sdk version

* Add milestones to the integration test

---------

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>