fix: unify local path and piped input

[flexdinesh]

• Keep checkout watching with servediff PATH; serve git diff | servediff in-process until Ctrl-C. Remove the pipe subcommand; local inputs ignore remote configuration.
• Reject path plus redirected stdin before startup. Preserve patch limits, retained review identity and singleton replacement; update help/docs/UI and rebuild embedded assets.
• Validation: mise run check:push passed in an isolated loopback network, including all 66 browser tests, Go/race, static/build/API/release checks and generated-asset drift.