CI: provision ripgrep for Home-tool benchmark gates

Problem

After #630 correctly passes the pinned workspace Home binary into build-backed gates, fresh Linux CI reaches that binary and fails three benchmark harness tools because rg is absent from the GitHub-hosted runner PATH: benchmark-comparison-harness.

The exact failures are home-tool: cannot spawn rg: FileNotFound in:

• tools/representative-benchmark.ts --self-test
• tools/test_representative_matrix.ts
• tools/representative-tier-attribution.ts --self-test

Required implementation

• Provision the Pantry rg package for matrix legs that build/run the pinned Home repository-tool runtime.
• Keep the tool dependency explicit in the workflow; do not replace semantic searches with ad hoc JavaScript scans or silently skip them.
• Preserve the exact pinned Home path from #630 and avoid workstation PATH assumptions.

Acceptance

• benchmark-comparison-harness executes all Home-backed self-tests without FileNotFound.
• threads-reference-audit, WebAssembly tool legs, and other home_tool matrix entries retain access to the same explicit dependency set.
• Workflow YAML and dependency inventory pass.

Parent: #630 Roadmap parent: #494 Related: #632

Implemented by b7db97b3.

The Home-enabled CI matrix legs now install Pantry-pinned rg@15.1.0, addressing the benchmark harness failure that occurred before sample collection. Local validation: workflow YAML parsed, dependency audit passed (15 classified edges, 0 migration targets, 60 classified repository tools), and this changes 0 Test262 cases.

Keeping this issue open until the fresh benchmark-comparison-harness job proves the pinned Home tool can complete its source inventory.

The first Pantry-input fix was insufficient. Fresh CI benchmark harness showed the pinned action resolve rg to crates.io/ripgrep and then skip it because the action bootstrap SDK only supports its small self-contained runtime set. The follow-up uses Pantry’s native canonical recipe resolver (pantry install crates.io/ripgrep@15.1.0 --no-save) and verifies rg --version before Home builds. This remains exact-version Pantry provisioning; no apt/curl fallback is involved.

c182b574 replaces the unsupported action-bootstrap package input with Pantry’s native canonical crates.io/ripgrep@15.1.0 resolver and verifies rg before Home builds. Fresh benchmark-harness CI will decide closure.

Resolved by c182b574. Fresh Linux CI confirms the benchmark comparison harness passes with the native Pantry crates.io/ripgrep@15.1.0 recipe, and the dependency audit remains green. The workflow executes rg --version before Home-backed legs, so a missing resolver result fails explicitly rather than skipping semantic searches.