[0.2.0v][Feature] PR indexing with dedicated collection and pr-duplicate CLI

[Kavirubc]

Overview

Extend the simili index command to optionally index pull requests into a dedicated Qdrant collection, and add a simili pr-duplicate CLI command to detect duplicate PRs against both issues and PRs.

Scope

• Add --include-prs flag to simili index to index PR metadata (title, description, changed file paths, linked issues) into a separate collection
• Add QDRANT_PR_COLLECTION / qdrant.pr_collection config option for the dedicated PR collection
• Add simili pr-duplicate CLI command to query both collections and run LLM duplicate detection
• processPullRequest worker: fetch PR details + file paths, build metadata text, embed and upsert
• Dry-run support for PR indexing

Acceptance Criteria

• simili index --include-prs indexes PRs into the configured PR collection
• PR collection is created automatically if it does not exist
• simili pr-duplicate --repo owner/repo --number 123 returns duplicate candidates from both collections
• Falls back gracefully if no PR collection is configured
• All existing issue indexing behaviour is unchanged
• Tests cover the new indexing and query paths

Notes

Extracted from PR #40. Implementation reference: nick1udwig/simili-bot@index-and-query-prs.
Depends on: #42 (OpenAI provider support) or can be implemented independently with Gemini only.