[Git Story] Scaffold Session Watcher responsibilities - #3129

Define the Session Watcher's responsibilities and data types, with pseudocode describing each step. The watcher gathers context from coding sessions and prepares it for memory processing. This PR adds the outline, not the working implementation.

• Capture new conversations, tool activity, and any edit diffs recorded in the transcript.
• Convert events into a common format while keeping their original IDs and the session they belong to.
• Collect session details such as the client, models, timestamps, and parent session.
• Connect a separate privacy filter before sending events and session details to memory processing. If filtering fails, do not send unfiltered data.
• Save the session ID, transcript file, last complete event ID, and read position so capture can resume. This tracks reading progress, not whether memory processing has finished.
• Leave memory creation, story writing, and deciding what belongs in each commit to other components.
• Add tests for the stubs and a short README explanation.

SessionWatchRequest + saved read checkpoint
    ↓
Read new transcript records
    ├── Save the next read checkpoint
    ↓
Convert events to a common format
    ↓
Package events with session details
    ↓
Apply privacy filtering
    ↓
Send approved content to memory processing

Validation: the package builds, all 10 tests pass, and formatting checks pass.