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/github.lua

@@ -12,6 +12,12 @@ local M = {
     pr_full = 'Pull Requests',
     ci = 'CI/CD',
   },
+  capabilities = {
+    draft = true,
+    reviewers = true,
+    per_pr_checks = true,
+    ci_json = true,
+  },
 }
 
 local function nwo()