watchpr: only alert on mergeability lost after the baseline

Track mergeability across the whole run and decode it as a tri-state, so a PR already conflicted when watching starts is polled on instead of reported.

internal/agent/gitea.go

@@ -189,12 +189,12 @@ func (c *GiteaClient) Whoami() (User, error) {
 
 // PullRequest is the subset of Gitea's PR object we track.
 type PullRequest struct {
-	Number    int    `json:"number"`
-	State     string `json:"state"`
-	Title     string `json:"title"`
-	Merged    bool   `json:"merged"`
-	Mergeable bool   `json:"mergeable"`
-	HTMLURL   string `json:"html_url"`
+	Number    int          `json:"number"`
+	State     string       `json:"state"`
+	Title     string       `json:"title"`
+	Merged    bool         `json:"merged"`
+	Mergeable Mergeability `json:"mergeable"`
+	HTMLURL   string       `json:"html_url"`
 	Head      struct {
 		Sha   string `json:"sha"`
 		Ref   string `json:"ref"`