Remove environment bridges and consolidate all inputs through typed configuration

[Cianidos]

Closes #138.

Removes the historical process-environment bridges that bypassed the typed
configuration system, so every user input resolves once into typed
run/workload/driver configuration and flows through native APIs. Environment
variables remain input sources (e.g. LOG_LEVEL, workload-param names), never
internal transport between command, bench, workload, and driver layers.

Changes:

• --steps/--no-steps are explicit bench.Run inputs; the bench step filter
  no longer reads STROPPY_STEPS/STROPPY_NO_STEPS.
• execute_sql SQL source is now the typed --sql-body / --sql-file
  workload params. Inline SQL, .sql path, and the sql positional bind as CLI
  inputs; STROPPY_SQL_BODY stays accepted as a legacy process-env alias.
• Legacy POOL_SIZE shorthand no longer post-applies after driver extras, so
  -D pool.maxConns / -D pool.minConns and the config-file pool block win
  deterministically.
• CSV workload identity comes only from the ?workload= URL option; the
  STROPPY_CSV_WORKLOAD fallback is removed.
• Dead STROPPY_DRIVER_N serialization/probe helpers and the generic
  pkg/bench env readers (Env/EnvInt/EnvFloat) are deleted.
• k6Args, k6Config, and driver-level defaultTxIsolation are removed from
  the v6 config schema and rejected; the docs schema and help topics are
  regenerated/updated.

Verification: make build, go test ./..., go vet ./..., and scoped
golangci-lint v2.12.2 over the changed packages are all clean.

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[Cianidos]

Blocked by skipped parent and config-contract decision

This PR cannot be mechanically retargeted to current main after #150 was skipped.

A dry rebase of only #151's commits (1e982655..e4ad1b50) onto current main stopped on the first commit with 14 conflicts. Most importantly, #151 modifies pkg/config, which exists only in #150; current main still uses the frozen protobuf config and protojson. Accepting the delete/modify conflicts would silently reintroduce #150 rather than isolate #151.

Retargeting also requires a functional policy decision for k6Args, k6Config, and defaultTxIsolation:

1. Keep them accepted but inert in the current protobuf contract.
2. Retain the protobuf API/descriptors but reject them at JSON/CLI boundaries.
3. Remove the fields, getters, descriptors, and schema entirely.

These options differ in compatibility, error timing, and public API. They are not mechanical conflict resolutions.

GitHub currently reports this PR clean only against refactor/issue-143-plain-go-config. Merging it as-is would update that blocked branch, not main; retargeting it directly to main would include all four #150 commits as well as this PR's three commits.

Please resolve #150's config policy, choose the legacy-field behavior above, then restack only the selected environment-bridge changes on current main. Per the sequential merge rule, #151 is skipped for now; no branch changes were pushed.