v26.06 source install flattens bwatch plugin path, but lightningd expects plugins/bwatch/bwatch

[marimes]

## Summary

After building and installing Core Lightning v26.06 from source on Ubuntu, `lightningd` failed during startup while registering the new `bwatch` plugin.

The installed file layout was:

```text
/usr/local/libexec/c-lightning/plugins/bwatch

but lightningd tried to register:

/usr/local/libexec/c-lightning/plugins/bwatch/bwatch

This caused:

UNUSUAL plugin-manager: failed to register plugin /usr/local/bin/../libexec/c-lightning/plugins/bwatch/bwatch

Build/install steps

Followed the documented Ubuntu source install flow:

uv sync --all-extras --all-groups --frozen
./configure
RUST_PROFILE=release uv run make
sudo RUST_PROFILE=release make install

Version:

v26.06

git describe showed:

v26.06-dirty

The dirty state appeared to be generated/vendor build artifacts, not a local source patch.

Observed install rule behavior

make -n install showed:

install ... plugins/bwatch/bwatch ... /usr/local/libexec/c-lightning/plugins

Since the destination is a directory, install flattened the source path and created:

/usr/local/libexec/c-lightning/plugins/bwatch

as a file.

However, at runtime CLN looked for:

/usr/local/libexec/c-lightning/plugins/bwatch/bwatch

Local workaround

This fixed startup:

sudo mv /usr/local/libexec/c-lightning/plugins/bwatch \
  /usr/local/libexec/c-lightning/plugins/bwatch.flat-installed

sudo install -d -m 755 /usr/local/libexec/c-lightning/plugins/bwatch

sudo install -m 755 /home/cln/lightning/plugins/bwatch/bwatch \
  /usr/local/libexec/c-lightning/plugins/bwatch/bwatch

After that:

lightningd --version

returned:

v26.06

without the plugin registration error.

Expected behavior

Either:

1. make install should preserve the nested plugin path for plugins/bwatch/bwatch, or
2. runtime plugin lookup should expect the flattened installed path /usr/local/libexec/c-lightning/plugins/bwatch.

Given the source tree defines bwatch as plugins/bwatch/bwatch, preserving the nested install directory seems likely to be the intended behavior.