with-watch: add first-class buf generate adapter

[kdy1]

Summary

Add a first-class buf generate adapter so with-watch can infer stable filesystem inputs for common Buf code-generation workflows without requiring exec --input for the normal happy path.

Evidence

• Issue with-watch: add first-class adapters for common search and schema/codegen tools #378 originally scoped buf generate alongside rg, ag, fd, protoc, flatc, thrift, and capnp compile.
• The initial adapter pass can ship the other commands safely with argv-only parsing, but buf generate often derives both inputs and outputs from buf.gen.yaml, buf.yaml, and command-line template/config overrides.
• This repository already uses buf generate in multiple places, including apps/dexdex/buf.gen.yaml, apps/devkit/buf.gen.yaml, and several server-side proto generation scripts.

Current Gap

• with-watch still falls back conservatively for buf generate, so normal codegen workflows either miss stable inputs or require explicit exec --input declarations.
• Unlike the first batch of adapters, buf generate commonly needs light config/template awareness to avoid watching generated output directories while still capturing module roots, template files, and config files.
• Leaving buf generate inside with-watch: add first-class adapters for common search and schema/codegen tools #378 would block shipping the safer argv-only adapter batch that is already implementable.

Proposed Scope

• Add a dedicated buf generate adapter that handles the default invocation (buf generate) plus file-backed --template and --config overrides.
• Watch stable Buf inputs such as the working module root, buf.gen.yaml, buf.yaml, file-backed template/config overrides, and explicit --path / --exclude-path roots when those narrow the generation scope.
• Filter output destinations inferred from -o/--output and file-backed template out fields so generated files do not become watched inputs.
• Keep inline YAML/JSON templates or other unsupported dynamic shapes on the existing fallback path with exec --input guidance rather than guessing.
• Update CommandAdapterId, long-help inventory, docs, and tests alongside the adapter.

Acceptance Criteria

• with-watch buf generate watches the stable Buf module/config inputs needed for default generation workflows.
• with-watch buf generate --template buf.gen.web.yaml and with-watch buf generate --config buf.yaml watch those file-backed overrides.
• Generated output directories supplied by -o/--output or template out fields are filtered from the watch set.
• Unsupported inline template/config forms remain fallback cases rather than silently guessed first-class behavior.
• with-watch --help and the with-watch docs list buf support once the adapter lands.

Test Scenarios

• with-watch buf generate
• with-watch buf generate --template buf.gen.web.yaml
• with-watch buf generate --config buf.yaml
• with-watch buf generate --path proto/foo
• with-watch buf generate -o gen
• Negative case: with-watch buf generate --template '{"version":"v2"}' remains a fallback case or fails with exec --input guidance.

Out of Scope

• Parsing arbitrary inline YAML/JSON templates in the first pass.
• Deep project-graph inference beyond the stable file/template/config inputs that Buf exposes directly.
• Expanding shell mode or changing the public CLI surface.