feat(playbooks): export playbooks as importable YAML, like personas #16084

Problem

Users can export a persona as a YAML file from the Agent Library ("Export YAML" in the persona menu, web and mobile). Playbooks have no equivalent. To share a playbook with a teammate, move it to another environment, or back it up, I have to find the file under .j5/playbooks/ by hand. Remote and mobile users often can't reach that directory at all.

Proposal

Add an Export YAML action to each playbook in the Playbook Library settings. It downloads <name>.yaml, and the file imports back through the existing playbook import without changes.

Behavior

• Exported YAML contains the full definition: title, description, and every step (id, title, prompt, plus persona where set).
• Exporting a playbook and importing it into another workspace gives an identical playbook.
• The file name follows the playbook name, so the existing import rules (PLAYBOOK_NAME_PATTERN, .yaml/.yml, 256 KiB limit) accept it.
• Export works for playbooks in remote environments. The server returns the YAML rather than the client reading the local filesystem, the same way readAgentPersona returns a YAML rendering and file name.
• Export is read-only, so no confirmation dialog.

Surfaces to cover

• Web: Export action in PlaybookLibrarySettings, reusing the Blob download pattern from exportPersona.
• Desktop: inherits web.
• Mobile: Export action in the playbook library screen, using the same share/save flow as persona export.
• Contracts: one read RPC on J5PlaybookRpcGroup, taking the PlaybookLibraryRequest shape plus a name and returning { fileName, yaml }.
• Connection modes: must work in local, remote/relay, and tunnel setups, since the server produces the content.
• Reverse state: import already exists on web. Confirm mobile can import playbooks, or file a follow-up so export isn't a one-way door there.

Out of scope

• Bulk or zip export.
• Exporting run history.
• Name-conflict resolution on import beyond what exists today.

Acceptance criteria

• Export YAML is available per playbook on web and mobile.
• A round trip (export, delete, import) restores the same definition.
• Server test covers the YAML rendering and file name; a client test covers any non-trivial logic.
• docs/user/ playbook section mentions export.