feat(gmail): add messages modify command for single-message label changes

[zerone0x]

Summary

Fixes #277

Adds gog gmail messages modify <messageId> --add <labels> --remove <labels> so users can add/remove labels on individual messages, not just threads.

Changes

• Add GmailMessagesModifyCmd to the messages subcommand group, mirroring the existing thread modify pattern
• Uses Gmail API Users.Messages.Modify for single-message label operations
• Supports --add and --remove flags with comma-separated label names or IDs
• Supports --dry-run, --json, and label name-to-ID resolution (consistent with existing commands)
• Add test covering JSON output, label resolution, and plain text output

Usage

# Trash a single message (without affecting other messages in the thread)
gog gmail messages modify <messageId> --add TRASH --remove INBOX

# Apply a custom label
gog gmail messages modify <messageId> --add "my-label"

# Dry run
gog gmail messages modify <messageId> --add STARRED --dry-run

Note

main currently has a pre-existing build error in contacts_crud.go (parameter type grouping bug). This PR does not touch that file. Tests will pass once that is resolved separately.

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🤖 Generated with Claude Code

[steipete]

Landed via temp rebase onto main.

• Gate: go test ./internal/cmd -run 'TestGmailMessagesModifyCmd_' -count=1, make ci
• Land commit: d195e1d
• Merge commit: d448264a27734e08ce9e666c8a20392911b4ee61

Fixups included:

• changelog entry
• shared label-ID resolution helper reused by message/thread/batch modify flows
• extra regression coverage for custom label name resolution and empty message IDs

Thanks @zerone0x!