Add memory tags autocomplete in CLI

[KD-K2N2]

Description

When using cortex add --tags, suggest existing tags from the project.

Expected behavior

Currently tags are free-text. Add a cortex tags command that lists all tags used in a project, so users can reference them.

$ cortex tags
architecture (12 memories)
typescript (8 memories)
deployment (5 memories)
deepgram (4 memories)
performance (3 memories)

Good first issue

Query: SELECT tags FROM memories WHERE project_id = ? AND deleted_at IS NULL. Parse JSON arrays, count occurrences, sort by frequency.