feat: implement cova status command

[MrPointer]

📝 Summary

Implements cova status — a read-only command that displays subscriptions, applied blocks, and configured agents. Reads from local config and state only, no network operations. Supports a default summary view and a verbose (-v) mode with per-subscription block breakdowns.

Also cleans up stale "Not yet implemented" doc notes for both cova status and cova remove.

🔑 Key Decisions

Read-only with no side effects

The Cobra layer checks if the state DB file exists before opening it. If missing, it passes a nil BlockStore to Run, which treats it as 0 applied blocks. This avoids creating state.db as a side effect of running a read-only command — unlike add/apply/remove which need the DB and create it on demand.

Output to io.Writer instead of Logger

Status output is the command's primary structured output, not a log message. The Deps struct accepts an io.Writer (stdout in prod, buffer in tests). Logger is used only for warnings (malformed state records, query failures).

Block deduplication

A single block can produce multiple file placements (e.g., SKILL.md + config.yaml). Records are deduplicated by (subscription, blockType, blockName) to count blocks, not files.

Default branch gap documented as tech debt

cova add doesn't resolve the default branch when --ref is omitted, so status can't show which branch a subscription tracks. Added a ROADMAP tech debt item rather than expanding scope.

🔄 Before & After

Area	Before	After
Status visibility	No way to inspect subscriptions/blocks without reading config YAML and state DB directly	cova status and cova status -v
cova remove docs	Still marked "Not yet implemented" despite being shipped	Docs match reality
State DB on first run	Any command creates state.db	cova status skips DB creation if file doesn't exist
Verbose output	N/A	Per-subscription block listing grouped by type, with summary lines
Worker agents	sonnet-go-worker missing applying-effective-go	Added, plus new sonnet-go-cli-worker for Cobra work