Add Jira connector

[alecgard]

Summary

• Add Jira Cloud connector (internal/connector/jira.go) with REST API v3 integration
• Support ingestion by project key (--jira PROJ) and/or arbitrary JQL (--jira-jql "sprint = 42")
• Incremental ingestion using updated > "YYYY-MM-DD HH:mm" JQL filter on re-ingest
• ADF (Atlassian Document Format) to plaintext conversion for descriptions and comments
• Rate limiting with exponential backoff on 429 responses, respecting Retry-After headers
• Configurable concurrency, page size, and metadata fields via env vars (KB_JIRA_*)
• Each issue becomes one RawDocument combining summary, description, comments, and metadata header

Closes #41

Test plan

• Factory validation tests (missing config fields, valid configs)
• Basic scan with project key
• Scan with JQL query
• Incremental scan with LastIngest timestamp
• Deletion detection on full scan
• Pagination across multiple pages
• ADF to plaintext conversion (paragraphs, headings, inline formatting, null/empty)
• Comment extraction
• Rate limit handling (retry on 429) and rate limit exceeded
• Auth header verification (Basic auth with email + API token)
• Checksum-based skip of unchanged issues
• Context cancellation
• Timestamp parsing
• make test passes

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