Use session IDs as task IDs for subagents instead of UUIDs

[tlongwell-block]

Replace UUID task IDs with session IDs to reduce LLM transcription errors.

The Problem

LLMs can mistype UUIDs when referencing tasks:

• a1b2c3d4-e5f6-7890-abcd-ef1234567890 → easy to corrupt
• 36 characters of random hex → higher error probability

The Solution

Session IDs are more LLM-friendly:

• 20251027_1234 → less likely to corrup
• 13 characters with a pattern → lower error probability

Result

Fewer failed task executions due to ID typos, improving Goose's reliability when orchestrating subagents.

Additionally, this task ID format is human-readable and meaningful

[tlongwell-block]

Resolves https://github.com/block/goose/issues/4955