feat(kanban): configure worktree paths and branches

Salvages #26496 by @aqilaziz. Adds branch_name column + CLI flag so
tasks with workspace_kind='worktree' can pin a target branch on
create. Schema migration added to _migrate_add_optional_columns.

- Task.branch_name field + DB column + migration
- create_task accepts branch_name kwarg
- hermes kanban create --branch <name> flag
- kanban show output includes 'Branch: <name>' when set

Cherry-picked the substantive commit (a7558cf27); the PR's tip was
an unrelated service-path-dirs commit. Resolved 2 INSERT-column-list
and show-output conflicts alongside main's session_id and
max_runtime_seconds additions; kept all three.

skills/devops/kanban-worker/SKILL.md

@@ -21,7 +21,7 @@ Your workspace kind determines how you should behave inside `$HERMES_KANBAN_WORK
 |---|---|---|
 | `scratch` | Fresh tmp dir, yours alone | Read/write freely; it gets GC'd when the task is archived. |
 | `dir:<path>` | Shared persistent directory | Other runs will read what you write. Treat it like long-lived state. Path is guaranteed absolute (the kernel rejects relative paths). |
-| `worktree` | Git worktree at the resolved path | If `.git` doesn't exist, run `git worktree add <path> <branch>` from the main repo first, then cd and work normally. Commit work here. |
+| `worktree` | Git worktree at the resolved path | If `.git` doesn't exist, run `git worktree add <path> ${HERMES_KANBAN_BRANCH:-wt/$HERMES_KANBAN_TASK}` from the main repo first, then cd and work normally. Commit work here. |
 
 ## Tenant isolation