Fix thread token parsing to prevent misidentifying payload tokens

[Copilot]

Thread tokens starting with "thread" followed by letters (e.g., "threading", "threads", "threadsafe") were incorrectly parsed as thread IDs instead of payload tokens.

Changes

• Validation function: _is_valid_thread_id() enforces thread IDs must start with digit, underscore, or hyphen
• Decode path: Validates extracted thread IDs before assignment; invalid tokens treated as payload
• Encode path: Validates thread_id at message creation with clear error messages
• Documentation: Wire format grammar, SlipMessage docstring, and examples updated

Thread ID Format

Valid (distinguished from English words):

slip("alice", "bob", "RequestReview", thread_id="42")       # thread42
slip("alice", "bob", "RequestReview", thread_id="_task")    # thread_task
slip("alice", "bob", "RequestReview", thread_id="-pr-123")  # thread-pr-123

Invalid (rejected at encode time):

slip("alice", "bob", "RequestReview", thread_id="task42")   # ValueError

Payload tokens correctly handled:

decode("SLIP v1 alice bob RequestReview threading data")
# thread_id=None, payload=["threading", "data"]

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