skills/book-gig: tailor pitch templates for returning venues and prior phone/in-person conversations

[JoshuaVSherman]

Child of web-jam-tools#875 ("book-gig skill phase 2 enhancements (Epic)").

What this builds

Enhances email pitch generation in book-gig (src/book-gig/pitch.ts and template selection) to tailor email intros, bodies, and tone based on relationship history:

1. Returning Venues (Played in the Past):
   • When a venue has prior gigs on record (priorGigs or relationshipStage === 'returning'), dynamically generates warm returning phrasing (e.g., "It's always great playing for you guys at [Venue]..." / "We loved playing for your crowd in the past and would love to return on [Weekend]...") instead of cold introductory boilerplate.
2. Prior Phone / In-Person Conversations:
   • When venue record notes or contact metadata indicates a prior conversation (contactNotes, priorContactNotes, or phone interaction), tailors the opening greeting and intro (e.g., "Hi [Contact], following up on our recent phone conversation / connecting in person...").
3. Template Fallback & Safety:
   • Strictly enforces cross-AI voice rules (no banned marketing buzzwords like "reach out" or "perfect fit").
   • Gracefully falls back to standard warm intro when no prior gig or conversation metadata is recorded.
4. Automated Unit Tests:
   • Adds unit test suite in test/book_gig_pitch.test.ts verifying returning-venue copy, spoken-conversation copy, and cold-pitch fallback paths.

Acceptance criteria

1. Pitches for returning venues reference past performances and express eagerness to return without generic intro boilerplate.
2. Pitches for contacts with recorded phone/in-person notes reference the prior conversation.
3. All rendered pitches remain compliant with voice rules.
4. deno task test, deno task fmt:check, deno task lint, and deno task sast pass 100% green.