FN-5843: scaffold standalone external-author plugins with reference template

Add a standalone external-author plugin scaffold flow and reference plugin output for fn plugin new.

- add standalone scaffold mode to plugin generator with external-author defaults
- generate reference plugin files and metadata tailored for out-of-tree/plugin-author usage
- update CLI wiring and command behavior for the new scaffold path
- expand CLI and scaffold tests to cover standalone generation behavior
- document the updated plugin scaffolding flow in CLI reference docs

Files changed:
 docs/cli-reference.md                              |   3 +-
 packages/cli/src/__tests__/bin.test.ts             |  12 +-
 packages/cli/src/__tests__/plugin-scaffold.test.ts | 114 ++++++---
 packages/cli/src/bin.ts                            |  18 +-
 packages/cli/src/commands/plugin-scaffold.ts       | 269 +++++++++++++++++++--
 5 files changed, 356 insertions(+), 60 deletions(-)

Fusion-Task-Id: FN-5843

Fusion-Task-Lineage: 4383e9ed-0b12-451e-a946-2d002f7e8faf

docs/cli-reference.md

@@ -971,9 +971,10 @@ fn plugin uninstall <id> --force
 fn plugin enable <id>
 fn plugin disable <id>
 fn plugin create <name>
+fn plugin new <name> [--output <dir>] [--scope <scope>]
 ```
 
-Subcommands: `list|ls`, `install`, `rescan`, `trust`, `untrust`, `verify`, `uninstall`, `enable`, `disable`, `create`.
+Subcommands: `list|ls`, `install`, `rescan`, `trust`, `untrust`, `verify`, `uninstall`, `enable`, `disable`, `create`, `new`.
 
 Scope semantics:
 - `fn plugin install` / `fn plugin uninstall` are **global** operations