Use appcmd.Command in changed-plugins

[unmultimedio]

Summary

• Migrates changed-plugins from the standard flag package to appcmd.Command, consistent with other commands in the repo
• Removes env-var coupling (BASE_REF, INCLUDE_TESTDATA) from plugin.FilterByBaseRefDiff — parameters are now passed explicitly
• Adds --base-ref (required), --dir (default .), and --include-testdata flags to the command
• Updates ci.yml and pr.yml to pass flags inline instead of injecting env vars

[unmultimedio]

Test script

test-changed-plugins.sh

#!/usr/bin/env bash
set -euo pipefail

# Pick any ref with known plugin changes. Default: the commit before
# connectrpc/go + connectrpc/gosimple were updated.
BASE_REF="${1:-4950ddf0~1}"

echo "==> Base ref: $BASE_REF"
echo

echo "==> changed-plugins output (space-separated):"
go run ./internal/cmd/changed-plugins --base-ref "$BASE_REF" | tr ' ' '\n' | sort
echo

echo "==> Unique plugin dirs touched (git cross-check):"
git diff --name-only "$BASE_REF" HEAD \
  | { grep '^plugins/' || true; } \
  | awk -F/ 'NF>=4 {print $2"/"$3":"$4}' \
  | sort -u

Produces output:

bash .test-changed-plugins.sh                                         
==> Base ref: 4950ddf0~1

==> changed-plugins output (space-separated):
2026/04/22 12:25:48 including plugin: plugins/connectrpc/go/v1.19.2/buf.plugin.yaml
2026/04/22 12:25:48 including plugin: plugins/connectrpc/gosimple/v1.19.2/buf.plugin.yaml
connectrpc/go:v1.19.2
connectrpc/gosimple:v1.19.2

==> Unique plugin dirs touched (git cross-check):
connectrpc/go:v1.19.2
connectrpc/gosimple:v1.19.2