converge: phase 2 — SSE relay client for realtime pull on push

- RepoConfig gains rcGiteaRepo :: Maybe Text for relay matching
- Options gains optRelayUrl :: Maybe Text (--relay-url CLI, relay_url config)
- connectRelay connects to converge-relay SSE endpoint, parses push events,
  triggers gitPull on matching repos (by gitea_repo + branch)
- Exponential backoff reconnection (1s .. 60s) on disconnect/error
- Skips pull if repo is mid-operation (merge/rebase/etc.)
- Dependencies: http-client, http-client-tls, http-types, aeson, bytestring
- SSE parser handles event:/data: fields, blank-line delimiters, CRLF endings
- Updated SPEC.md, example config, and help text

SPEC.md

@@ -18,3 +18,8 @@
 ## Can notify on pull: per-repo notify_on_pull setting sends a desktop notification when new commits are pulled from the remote
 
 ## --sync / -s flag runs a single sync cycle (commit + pull + push) and exits, useful for wake-from-sleep hooks
+
+## Can connect to a converge-relay SSE endpoint and automatically pull when push events arrive
+### Configured via --relay-url CLI flag or relay_url config file entry
+### Matches repos by gitea_repo field (Gitea repository full_name) and branch
+### Reconnects automatically with exponential backoff on disconnect or error