feat: implement validate command

[Pertempto]

Summary

Implements the validate command from the MVP spec (000-basic-cli.md).

Changes

New Files

• internal/validate/parser.go - Goldmark-based spec parser with frontmatter extension
• internal/validate/parser_test.go - Tests for spec parsing
• internal/validate/validator.go - Validation logic for frontmatter, status, title, and task lists
• internal/validate/validator_test.go - Tests for validation
• cmd/validate_test.go - Integration tests for the validate command

Updated Files

• cmd/validate.go - Full implementation replacing stub
• specs/000-basic-cli.md - Marked validate command tasks as complete

Features

• Parse specs using goldmark with frontmatter extension
• Validate frontmatter - checks for presence and required status field
• Validate status - must be one of: draft, approved, in-progress, completed, rejected
• Validate title - checks for H1 heading
• Validate task list - checks for checkbox items
• Single-spec mode - specture validate 000 or specture validate specs/000-mvp.md
• All-specs mode - specture validate validates all specs in specs/
• Summary output - shows "X of Y specs valid"
• Clear error messages - actionable output for validation failures

Testing

just test  # All tests pass

Manual testing:

just build
./specture validate           # Validates all specs
./specture validate 000       # Validates specific spec by number
./specture v                  # Alias works