Release Planr 1.8.0 with first-class Grok Build integration

[regenrek]

Why

• Add explicit, portable, local-first Grok Build support without xAI credentials or live inference in CI.
• Prepare the reviewed Planr 1.8.0 release candidate and immutable release gates.

How

• Add Grok host rendering, installer surfaces, contracts, docs, and compatibility tests.
• Bind external eval evidence to the exact 300-file candidate source and the evaluated workflow subset.
• Harden release, changelog, portability, workflow, cancellation, and deterministic concurrency gates.

Tests

• cargo fmt --check && cargo clippy --all-targets --all-features -- -D warnings
• cargo test -- --test-threads=1 (260 passed)
• npm run verify:release-eval-gate (1 positive, 12 negative)
• npm run verify:release-script && npm run verify:github-actions
• npm run docs:verify-reference && npm run docs:verify-release
• npm run pack:check && npm run security:privacy
• scripts/verify-changelog-release-links.sh 1.8.0
• target/debug/planr install grok --dry-run