Add plugin init scaffold to pipectl

[kubeboiii]

What this PR does:

Adds pipectl plugin init and the pluginscaffold package to generate compile-ready Piped v1 plugin skeletons from embedded templates:

• Stage-only layout (based on the wait plugin)
• Deployment-minimal layout (based on a slim ecs-style layout)

The command supports --kind, --stages, --module, --name, --dry-run, and --force. Tests include golden output, name validation, and integration checks that run go mod tidy and go build for both kinds.

Why we need it:

Plugin authors currently copy and rename files from existing plugins (wait, ecs), which is slow and easy to get wrong. This addresses the MVP in #6694 and part of the scaffold epic in #6693.

Which issue(s) this PR fixes:

Fixes #6694

Related to #6693 (does not implement the full epic; see scope below).

Does this PR introduce a user-facing change?:

• How are users affected by this change: Plugin authors can run pipectl plugin init to create a new plugin directory with a valid layout and stub implementations. Documentation is added under Contributing plugins and Command-line tool.
• Is this breaking change: No
• How to migrate (if breaking change): N/A

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Scope

In scope	Out of scope (follow-up)
pipectl plugin init	pipectl plugin add-stage (#6705)
Stage + deployment-minimal templates	--livestate, --planpreview, --initialize (#6693)
Community module path default	In-tree pkg/app/pipedv1/plugin/... helper

This change is additive: piped runtime and existing plugins are not modified. pipectl plugin push is unchanged.

The diff is larger than the ~300-line guideline in CONTRIBUTING because of embedded templates and golden testdata; it is limited to one feature (the init scaffold).

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Test plan

• make check
• go test -race ./pkg/app/pipectl/pluginscaffold/... ./pkg/app/pipectl/cmd/plugin/...

make build/go MOD=pipectl

./.artifacts/pipectl plugin init /tmp/demo-stage \
  --kind stage --stages DEMO_WAIT
(cd /tmp/demo-stage && go mod tidy && go build .)

./.artifacts/pipectl plugin init /tmp/demo-deployment \
  --kind deployment --stages DEMO_SYNC,DEMO_ROLLBACK
(cd /tmp/demo-deployment && go mod tidy && go build .)

./.artifacts/pipectl plugin init /tmp/demo-dry-run \
  --kind stage --stages DEMO_WAIT --dry-run