fix: WorkCompletionLearning reads nonexistent state files

[qozle]

Fixes #972

Problem

WorkCompletionLearning.hook.ts never generates learning files due to two bugs:

1. findStateFile() looks for files that don't exist. It searches for MEMORY/STATE/current-work.json or current-work-{sessionId}.json, but nothing in PAI creates these files. The hook always exits with "No active work session."

2. ISC regex doesn't match PRD v2.0 format. The regex looks for ## IDEAL STATE CRITERIA but PRDFORMAT.md v2.0 defines the section as ## Criteria. ISC content is never extracted.

There's also a field name mismatch: PRD frontmatter uses task/slug/started but the WorkMeta interface expects title/id/created_at.

Fix

1. Replace findStateFile() with findWorkSession() that reads work.json (the actual state file, populated by PRDSync hook) and matches sessions by sessionUUID.

2. Update the ISC regex to accept both ## IDEAL STATE CRITERIA (legacy) and ## Criteria (v2.0).

3. Add field mapping in parseYaml() so v2.0 frontmatter fields get mapped to the interface fields the rest of the hook expects.

Verification

• Confirmed PRDSync.hook.ts writes to MEMORY/STATE/work.json via syncToWorkJson() in prd-utils.ts
• Confirmed prd-utils.ts countCriteria() uses ## Criteria header (line 61), matching the fix
• Confirmed work.json registry keys sessions by slug (= the WORK directory name), so session_dir mapping is correct