feat: add 'legal' as a task cluster pattern

[iRonin]

Adds a legal category to the prompt pattern classifier.

With the current classifier, 'other' is the largest category at ~30% of sessions. The new legal cluster captures litigation-related sessions (affidavits, exhibits, court filings, ECSC, etc.) that currently fall through to 'other'.

Keywords matched (case-insensitive):

• affidavit, exhibit, pleading, brief, court, filing
• cpr, defendant, claimant, injunction, discovery, deposition
• ecsc, application notice, witness statement, costs

Placed after 'git ops' so it matches before 'other'.

Closes #11.