Align Grok plugin manifest with the 0.8.1 release identity

[iammedved]

Proposal

README, the Codex plugin, and CHANGELOG already declare 0.8.1, but .grok-plugin/plugin.json on main is still 0.8.0. There is no v0.8.1 tag, so codex plugin marketplace add iammedved/Pinmind --ref v0.8.1 cannot work.

This PR is the identity slice only: bump the Grok manifest, keep ROADMAP's public baseline in step, and make the existing coherence test fail if Codex and Grok versions drift again.

Observed RED before the bump: 0.8.0 !== 0.8.1 in tests/kernel.test.mjs. GREEN after the bump.

Safety checklist

• I removed secrets, credentials, personal data, private URLs, and local absolute paths.
• I included only files needed for this proposal.
• I ran the relevant validation commands.
• I understand that submission does not grant write or merge access and that the maintainer decides whether to apply the change.

Verification

• node --test tests/kernel.test.mjs tests/release-verification.test.mjs — 71/71 pass
• node scripts/validate-plugin-skill.mjs — ok, version 0.8.1

Tag v0.8.1 and the GitHub Release stay after merge.