feat: add capabilities system and per-forge compatibility

Problem: forge.nvim silently ignored unsupported features on
non-GitHub forges. Codeberg `pr_for_branch_cmd` blocked all PR
creation, CI picker had zero actions, `repo_info` was hardcoded,
and the compose buffer showed draft/reviewer fields that did nothing.

Solution: add `forge.Capabilities` declaration (`draft`, `reviewers`,
`per_pr_checks`, `ci_json`) to each source. Compose buffer hides
unsupported fields. Per-PR checks falls back to repo-wide CI with
a notification. Fix Codeberg `pr_for_branch_cmd` to filter by branch
via jq, implement `repo_info` and `list_runs_json_cmd` via Gitea API,
add `default_branch_cmd` fallback, and add yank notifications for
GitLab/Codeberg.

lua/forge/pickers.lua

@@ -175,7 +175,14 @@ local function pr_actions(f, num)
     {
       name = 'ci',
       fn = function()
-        M.checks(f, num)
+        if f.capabilities.per_pr_checks then
+          M.checks(f, num)
+        else
+          require('forge').log(
+            ('per-%s checks unavailable on %s, showing repo CI'):format(kind, f.name)
+          )
+          M.ci(f)
+        end
       end,
     },
     {