CLI: --tag flag appends instead of replacing tags

[orveth]

Bug

When using blaze edit <id> --tag foo --tag bar, the tags are appended to existing tags rather than replacing them.

Steps to Reproduce

1. Create a card with tags: blaze add "Test" --tag old-tag
2. Edit to change tags: blaze edit <id> --tag new-tag
3. Check the result: blaze show <id> -o json | jq '.tags'

Expected Behavior

Tags should be replaced, or there should be explicit --tag +add / --tag -remove syntax for incremental changes.

Actual Behavior

Tags accumulate. Example result after multiple edits:

["blaze", "nostr,infrastructure", "nostr", "infrastructure"]

Also note that --tag nostr,infrastructure was treated as a single tag string rather than two separate tags.

Suggested Fix

Either:

1. Make --tag replace all tags (simpler)
2. Add --tag +foo to add and --tag -foo to remove (more flexible)
3. Add --clear-tags flag to reset before adding