feat: remove updated field from frontmatter

[jrepp]

Summary

• Remove updated field from frontmatter schemas - it can be derived from git history
• Add bulk commands for managing frontmatter fields (update, timestamps, migrate)
• Migrate from legacy date field to created datetime format
• Normalize created field to ISO 8601 datetime format from git history
• Remove deprecated project-specific fix modules (broken_links, mdx_code_blocks, etc.)
• Add comprehensive test coverage for new functionality

Usage

# Update timestamps for all documents (sets created from git history)
docuchango bulk timestamps

# Update only ADR timestamps
docuchango bulk timestamps --type adr

# Preview changes without applying
docuchango bulk timestamps --dry-run

# Migrate documents (removes updated/date fields, adds created)
docuchango bulk migrate

# Bulk update a specific field
docuchango bulk update --field status --value Draft --operation set

Changes

• Schemas: Removed updated field from ADR, RFC, Memo, and PRD schemas
• Templates: Updated templates to use created instead of date/updated
• CLI: Added bulk command group with update, timestamps, and migrate subcommands
• Timestamps: bulk timestamps now only manages created field
• Migration: bulk migrate removes updated and date fields, adds created from git

Test plan

• All existing tests pass (554 passed)
• New tests added for bulk commands and timestamp functionality
• Verified updated field is no longer populated during bulk operations

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