Support tags on items and tag-scoped search/filtering

[kacheo]

Problem

Tags currently live only on day files. Blockers and action items can't be tagged, so you can't group or filter them by area (e.g. auth, infra).

Proposal

• Let devlog add --section blockers/action_items accept --tag, storing tags on the item in items.yaml.
• Add a --tag filter to devlog items.
• Extend devlog search to support tag-scoped queries.

Note

Tag validation already exists (^[a-z0-9_]+$); reuse it for items so the tag namespace stays consistent across day files and items.