fix: prefer local patched imsg binary

[kacy]

what changed

this teaches the bridge how to resolve imsg from a few override points instead of assuming the binary only comes from PATH.

it now prefers:

• the explicit -imsg-bin flag
• IMSGBRIDGE_IMSG_BIN
• a sibling checkout at ../imsg/bin/imsg when that exists
• plain imsg on PATH as the fallback

make run also passes the resolved local binary path so local bridge work will pick up a patched checkout automatically.

why

history performance work currently lives in a local imsg fork, not upstream. without an override, the daemon keeps using the installed imsg binary and misses that faster history path.

impact

local development can point the bridge at a patched imsg build without editing code each time.

production behavior stays the same unless an override is provided or the sibling checkout layout is present.

validation

• go test ./...