feat: initial implementation of LLM RSS aggregator with DeepSeek summarisation

- Add core aggregation engine (`internal/aggregator/`)
  - Parse RSS/Atom/JSON feeds using `gofeed`
  - Extract article metadata (title, link, date, author)
  - Fallback to web scraping with `goquery` for minimal content
- Add content processing and filtering (`internal/processor/`)
  - Keyword-based include/exclude filtering
  - Sort by date, title, or source
  - Token estimation and context preparation
- Add DeepSeek API integration (`internal/llm/`)
  - OpenAI-compatible client for chat completions
  - Configurable model, max tokens, temperature
  - Support custom system prompts
- Add command-line interface (`internal/cli/`)
  - `go-arg` based argument parsing
  - Options for feeds file, prompt, filtering, output formats
  - Environment variable support for API key
- Add output formatting (`internal/output/`)
  - Plain text, GitHub‑flavoured markdown, and JSON outputs
  - Optional inclusion of original articles
- Add terminal user interface (`internal/tui/`)
  - Bubble Tea based progress bar with gradient colours
  - Live article counters and elapsed time
  - Keyboard controls (q/Ctrl+C to quit)
- Add runtime orchestration (`internal/runtime/`)
  - Pipeline execution (aggregate → process → summarise → output)
  - File output support
- Add project documentation
  - `README.md` with usage examples and dependencies
  - `CHANGELOG.md` for version 0.1.0
  - `LICENCE.txt` (EUPL 1.2)
  - Third‑party library READMEs in `docs/` for reference
- Add build configuration
  - `go.mod` with dependencies (gofeed, openai-go, bubbletea, lipgloss,
    go-arg, goquery)
  - `.goreleaser.yaml` for cross‑platform releases
  - `.gitignore` for Go binaries and test artifacts