Add YAML input support (-I yaml)

[vmvarela]

Description

Add YAML as a supported input format. Parse YAML files containing lists of maps as tabular data.

# users.yaml
- id: 1
  name: Alice
  balance: 250.50
- id: 2
  name: Bob
  balance: 150.75

sql-pipe -I yaml users.yaml 'SELECT SUM(balance) FROM t'
# Auto-detect from .yaml/.yml extension:
sql-pipe users.yaml 'SELECT * FROM t WHERE id > 1'

Motivation

YAML is ubiquitous in CI/CD (GitHub Actions, GitLab CI), configuration files (Docker Compose, Kubernetes, Ansible), and infrastructure-as-code. It is the biggest format gap in sql-pipe's input support.

Acceptance Criteria

• Add yaml to the InputFormat enum
• Auto-detect YAML from .yaml and .yml file extensions
• Parse YAML list-of-maps as rows with map keys as column names
• Support nested YAML structures (flatten like JSON does)
• Type inference works on YAML values (int, float, bool, null, string)
• -I yaml explicit override works for stdin and non-standard extensions
• Integration tests with various YAML structures
• Help text updated

Implementation Notes

• Requires a YAML parser — options: bundle libyaml (C library, like SQLite amalgamation) or implement a minimal Zig parser
• YAML is complex (anchors, aliases, multi-line strings, flow vs block style) — bundling libyaml is the pragmatic choice
• The existing JSON flattening logic can be reused for nested structures
• Estimated effort: 1-2 days