Add rewritten awk scenario harness

[matt-dz]

Summary

Adds the first rshell-owned AWK scenario rewrite harness on top of the external AWK harness work in #233.

• Adds tests/awk_scenarios/enabled.txt as the explicit run list for rewritten AWK tests.
• Adds tests/awk_scenarios/upstream-map.yaml as an audit ledger mapping upstream gawk/BWK coverage to local rewritten tests.
• Adds a Go AWK scenario runner that validates enabled scenario metadata and runs enabled rewrites against a pinned GNU awk oracle or an AWK_UNDER_TEST candidate.
• Adds the first 9 original rewritten AWK scenarios for early GNU awk behavior: BEGIN/END, fields, -v, arithmetic/comparison, field rebuilding, split, print separators, regex matching, and BWK-style pattern/action behavior.
• Wires tools/awk-harness/run.sh rewritten, make test_awk_rewritten, CI bootstrap validation, and the implement-awk skill.

Validation

• make fmt
• go test ./tests -run 'TestAwkScenarioMetadata|TestAwkScenarios' -count=1
• GAWK_ORACLE=.../gawk-5.4.0/bin/gawk tools/awk-harness/run.sh rewritten
• go test ./...

Notes

This is stacked on #233 and targets codex/awk-bwk-tests so the diff only shows the rewritten-test harness layer.