refactor: scanner as library + CLI, remove issue posting

[packet-mover]

Summary

Scanner becomes a pure library (scan + format). Issue posting moves out - callers (like Codatus App) handle it via their own GitHub client.

• Removed `Run()`, `Config.ReportRepo`, and `CreateIssue` from the `GitHubClient` interface
• Added functional options to `Scan()`: `WithBaseURL(url)` lets callers point the scanner at a custom API (useful for WireMock in integration tests or GitHub Enterprise)
• Refactored `Scan(ctx, cfg Config, opts ...Option)` - constructs a client from `cfg.Token` and options, delegates to internal `scanWithClient`
• CLI now reads `CODATUS_ORG` + `CODATUS_TOKEN` only, prints the Markdown report to stdout, logs scan summary to stderr. Pipeable: `codatus > report.md`

Test plan

• `go test ./...` - all tests pass
• `go vet ./...` clean
• Scanner tests updated to call internal `scanWithClient` (preserves mock client usage)
• Removed `TestCreateIssue_*` tests (no longer relevant)