feat: add GNAP coordinator components for git-native multi-agent coordination

[SyedShahmeerAli12]

Summary

Closes #10829

Adds two new Haystack components implementing the GNAP (Git-Native Agent Protocol)
coordination pattern proposed in #10829:

• GNAPTaskCreator — writes a task as a markdown file into board/todo/ and
  optionally records the transition as a git commit
• GNAPResultReader — polls board/done/<task_id>.md until a result is written
  by another agent, then returns its content

Both components live in a new haystack/components/coordinators/ module and follow
all existing Haystack conventions: @component decorator, to_dict/from_dict
serialization, and lazy imports via LazyImporter.

Usage example

from haystack.components.coordinators import GNAPTaskCreator, GNAPResultReader

creator = GNAPTaskCreator(board_path="/path/to/board")
reader  = GNAPResultReader(board_path="/path/to/board", timeout=300)

# Pipeline A creates a task
task_id = creator.run(query="Summarize this document", task_spec={"priority":
"high"})["task_id"]

# A parallel agent writes board/done/<task_id>.md ...

# Pipeline B reads the result
output = reader.run(task_id=task_id)["result"]

Test plan

- 8 unit tests for GNAPTaskCreator (file creation, content, git mock, serialization, unique   
IDs)
- 5 unit tests for GNAPResultReader (immediate read, timeout, threaded wait, serialization)   
- All 13 tests pass

[vercel]

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