Declare tenacity + document retry policy

[nerualauren]

Two small housekeeping follow-ups to the retry-hardening work (#235).

1. Declare tenacity in requirements

embedding_utils imports tenacity but it was never declared in requirements.txt. Same latent-break class as the SDK-9 pinecone bug: works today only because tenacity is transitively present, would break a clean install. Pinned >=8.0.0 (floor where all the imported names exist).

2. Add CLAUDE.md documenting the retry policy

Records the policy the #235 consolidation established, so the next contributor doesn't re-fragment it:

• simple stateless transient retry -> common/retry.py:with_retry
• Pinecone 429 ladder -> bespoke with_pinecone_retry
• stateful (DB rollback between attempts) -> bespoke loops

Plus a note to grep for existing variants before adding a new util (parsimony pass per change).

Needs a one-line .gitignore negation (!CLAUDE.md) since the repo ignores *.md, mirroring the existing !requirements.txt.

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