feat: add --json-path flag to navigate nested JSON arrays for input

[vmvarela]

Summary

• Adds --json-path <dot.separated.path> flag for -I json mode so users can reach a nested array inside a JSON document
• Works for query mode, --columns, and --validate
• Omitting the flag preserves existing behaviour (bare top-level array expected)

Examples

# Single-key path
printf '{"data":[{"name":"Alice"},{"name":"Bob"}]}' | sql-pipe -I json --json-path data 'SELECT name FROM t'

# Multi-segment path
curl https://api.example.com/feed | sql-pipe -I json --json-path results.items 'SELECT title FROM t'

# Schema inspection on nested doc
printf '{"feed":{"entry":[{"title":"T1"}]}}' | sql-pipe -I json --json-path feed.entry --columns

Changes

• src/args.zig — MissingJsonFlagValue error, json_path field in ParsedArgs/ColumnsArgs/ValidateArgs, flag parsing, usage text
• src/json.zig — pub navigateJsonPath() helper, json_path parameter in loadJsonArray
• src/main.zig — thread parsed.json_path + error handler
• src/modes/columns.zig / src/modes/validate.zig — navigate via json_mod.navigateJsonPath
• build.zig — integration tests 132–136 (single-key, multi-segment, missing key, non-array, --columns)

Closes #141