Option 3: move watch internals behind planner

[rsanheim]

Summary

Implements the broader watch-boundary option. The watch package moves under internal/watch, and watch handling separates planning from execution so only explicit runnable plans are executed.

What changed

• Moved watch/ to internal/watch/ with no compatibility wrapper.
• Updated root command/runtime imports to use the internal watch implementation.
• Added explicit batch planning for runnable jobs, missing targets, no-target jobs, and no-runnable-path logging.
• Fixed the missing-only matched job bug while preserving explicit no_targets behavior and prior No existing targets for file debug output.

Validation

• git diff --check
• go test -mod=mod ./...
• bin/rake build
• PLUR_BINARY="$PWD/plur" bin/rspec spec/integration/watch/watch_config_spec.rb spec/integration/watch/watch_integration_spec.rb spec/integration/watch/watch_spec.rb spec/integration/watch/mismatched_dirs_spec.rb

Notes

This is the larger implementation option. It fixes the bug and removes the public watch package surface that exposed internal framework types.