docs(readme): drop stale plugins feature-flag mention

[yattdev]

Summary

• Removes the stale instruction to enable a plugins feature flag before installing a plugin. Plugins ship in the base product with no feature flag to enable first.
• This change is limited to README.md; the release-workflow rename hardening remains a separate follow-up because this credential cannot modify .github/workflows/**.

Review evidence

At commit f616642516372091f1a0aa6239bb693d3eb2839d:

grep -rn 'features\.plugins\|plugins feature flag\|`plugins` feature flag' README.md
go mod tidy && git diff --exit-code -- go.mod go.sum
test -z "$(gofmt -l .)"
make vet
make test
make package

The grep produces no matches; all remaining commands pass. manifest.yaml and Makefile both declare version 0.1.0.

The repository defines unconditional pull_request workflows for main: CI (tidy, format, vet, test) and Build (package). Neither has a README path filter, so checks should be present for this PR. GitHub currently shows neither check for the head; a maintainer needs to approve/re-run the fork PR workflows before merge. The only reported reviewer result is the neutral Cubic summary; no actionable review comments are present.

Deferred follow-up

.github/workflows/release.yml still hardcodes kandev-plugin-template in the README metadata rewrite, checksum extraction, and release-asset glob. That fix is intentionally not claimed by this README-only PR and needs a credential with the GitHub workflow scope.

Linked Kandev docs/guard PR: kdlbs/kandev#2309

[yattdev]

cc @jcfs @carlosflorencio

[yattdev]

The unchanged README-only head f616642516372091f1a0aa6239bb693d3eb2839d is still mergeable. Its CI run and Build run both became terminal failures on September 4 without creating any jobs. They were originally awaiting external-contributor workflow approval; there is no failing source-test log to repair.

Could a repository maintainer approve and rerun these workflows at the unchanged head? The contributing account has read-only base-repository access. The release-workflow ID derivation remains a separately deferred change and is not part of this README PR.