fix: harden GitHub fetcher and history I/O, add canonical keying

GitHub fetcher (github.go):
- add 30s HTTP client timeout (was http.DefaultClient with no bound)
- chunk GraphQL alias requests at 50 repos to stay clear of abuse detection
- abort the run on any partial GraphQL error or missing repo rather than
  silently shrinking the README and poisoning the next delta
- retry transient failures (network, 5xx, 429) with 2s/4s/8s backoff

History layer (history.go):
- key snapshots by canonical owner/repo from agents.yml instead of the
  rename-resolved NameWithOwner returned by the API; carry a lazy
  migration map so existing aaif-goose/goose entries fold into block/goose
  on next read with no manual data edit
- tighten the 7d delta window to (cutoff-3d, cutoff] so a missed cron week
  no longer mislabels a 90d-old comparison as Delta7d
- replace the snapshots[:0] aliased filter loop with slices.DeleteFunc
- log malformed JSONL lines to stderr with line numbers instead of
  silently skipping them
- write history.jsonl atomically via tmp file + rename so a crash
  mid-write can no longer truncate accumulated history

Plus collapse a few redundant fmt.Errorf wraps, drop a named Config type
that was used once, inline the single-call sortByStars helper with a
deterministic tiebreaker on canonical key, and use filepath.Base instead
of hand-rolling a basename.

Includes unit tests covering the 7d window edges, canonical-key
migration, atomic write path, malformed-line tolerance, YAML validation,
and markdown cell escaping.

agents.go

@@ -14,20 +14,17 @@ type Agent struct {
 	Notes    string `yaml:"notes,omitempty"`
 }
 
-type Config struct {
-	Agents []Agent `yaml:"agents"`
-}
-
 func loadAgents(path string) ([]Agent, error) {
 	data, err := os.ReadFile(path)
 	if err != nil {
 		return nil, err
 	}
-	var cfg Config
+	var cfg struct {
+		Agents []Agent `yaml:"agents"`
+	}
 	if err := yaml.Unmarshal(data, &cfg); err != nil {
 		return nil, err
 	}
-	// validate each entry has owner + repo
 	for i, a := range cfg.Agents {
 		if a.Owner == "" || a.Repo == "" {
 			return nil, fmt.Errorf("entry %d missing owner or repo", i)