feat(cmd): pagination unification + fj api --paginate

Before this, only `release list` walked pages. `repo list`, `pr list` (the
non-filter branch), and `issue list` all passed `PageSize: limit` directly
to the gitea SDK — which silently caps PageSize at 50, so any request for
more than 50 results was truncated to 50 with no warning. `--limit` was
effectively a per-page hint, not a real limit.

## Changes

- New `cmd/paginate.go` — generic `paginateGitea[T any]` that walks pages
  until the response is short or the limit is reached. Uses Go 1.20
  generics so each list command keeps its existing typed slice without
  conversion overhead.

- `repo list` — paginates ListUserRepos.
- `pr list` — paginates ListRepoPullRequests in both branches:
  - With client-side filters (assignee, author, labels, search, draft,
    head, base): pull all pages then filter+limit.
  - Without filters: paginate up to limit.
- `issue list` — paginates ListRepoIssues. Overshoots 2x because the API
  returns both issues AND PRs and we filter PRs out client-side; the
  overshoot keeps us bounded but reduces the chance of returning fewer
  results than `--limit`.

## `fj api --paginate`

Mirrors `gh api --paginate`:
- Follows RFC 5988 `Link: rel="next"` headers (Forgejo emits these on
  list endpoints).
- Concatenates each page's JSON array into a single array via
  `concatPaginatedJSON`. If a page is not a JSON array, errors with a
  clear message — `--paginate` only makes sense for paginatable endpoints.
- GET-only (errors on POST/PUT/DELETE).
- Reuses the same auth and custom headers across pages; the body-size
  limit applies per-page.

Refactored the request execution into a `doOnce` closure so the loop body
isn't a copy of the single-request path.

Verified live:

  $ fj api 'repos/public/claude-code-proxy/commits?limit=2' \
        --paginate --jq '. | length'
  44

(44 = total commits in the repo, walked via Link headers from a 2-per-page
starting query.)

Out of scope for this commit, deferred:
- De-duplicating cmd/aliases.go ↔ cmd/actions.go subtrees (the type
  mismatch they caused is already fixed in the prior commit; the
  duplication itself is polish).

cmd/pr.go

@@ -252,39 +252,32 @@ func runPRList(cmd *cobra.Command, args []string) error {
 	needsClientFilter := assignee != "" || author != "" || len(labels) > 0 || search != "" || draft || head != "" || base != ""
 
 	ios.StartSpinner("Fetching pull requests...")
+	// When client-side filtering is needed, pull pages until exhausted (no
+	// limit) so we can apply filters; otherwise paginate up to the user's
+	// limit. Either way, paginate — `PageSize: limit` capped at 50 silently.
+	fetchPage := func(page, pageSize int) ([]*gitea.PullRequest, error) {
+		batch, _, err := client.ListRepoPullRequests(owner, name, gitea.ListPullRequestsOptions{
+			State:       stateType,
+			ListOptions: gitea.ListOptions{Page: page, PageSize: pageSize},
+		})
+		return batch, err
+	}
 	var prs []*gitea.PullRequest
 	if needsClientFilter {
-		page := 1
-		for {
-			batch, _, err := client.ListRepoPullRequests(owner, name, gitea.ListPullRequestsOptions{
-				State:       stateType,
-				ListOptions: gitea.ListOptions{Page: page, PageSize: 50},
-			})
-			if err != nil {
-				ios.StopSpinner()
-				return fmt.Errorf("failed to list pull requests: %w", err)
+		prs, err = paginateGitea(0, fetchPage) // pull all, then filter + limit
+		if err == nil {
+			prs = filterPRs(prs, author, assignee, labels, search, draft, head, base)
+			if limit > 0 && len(prs) > limit {
+				prs = prs[:limit]
 			}
-			prs = append(prs, batch...)
-			if len(batch) < 50 {
-				break
-			}
-			page++
-		}
-		prs = filterPRs(prs, author, assignee, labels, search, draft, head, base)
-		if len(prs) > limit {
-			prs = prs[:limit]
 		}
 	} else {
-		prs, _, err = client.ListRepoPullRequests(owner, name, gitea.ListPullRequestsOptions{
-			State:       stateType,
-			ListOptions: gitea.ListOptions{PageSize: limit},
-		})
-		if err != nil {
-			ios.StopSpinner()
-			return fmt.Errorf("failed to list pull requests: %w", err)
-		}
+		prs, err = paginateGitea(limit, fetchPage)
 	}
 	ios.StopSpinner()
+	if err != nil {
+		return fmt.Errorf("failed to list pull requests: %w", err)
+	}
 
 	if wantJSON(cmd) {
 		return outputJSON(cmd, prs)