docs document BASE_CLI_TEMP_RETENTION_DAYS which doesn't exist in code

[codeforester]

Problem

docs/runtime-environment.md:207 and docs/local-config.md:101 both document BASE_CLI_TEMP_RETENTION_DAYS as a recognized, user-tunable variable ("Retention window for pruned temp directories"). It does not exist anywhere in the codebase.

Checked cli/python/base_cli_adapters/config.py (the actual env-var loader for base_cli.App config, lines 249-258) -- it reads exactly three variables: BASE_CLI_ENVIRONMENT, BASE_CLI_LOG_LEVEL, BASE_CLI_KEEP_TEMP. A repo-wide grep across .py and .sh files found no handling of BASE_CLI_TEMP_RETENTION_DAYS anywhere. The only "retention" concept that does exist in the codebase is basectl clean's unrelated log-retention/keep-count logic in cli/python/base_clean/engine.py, which is a different mechanism entirely (a CLI flag, not this env var).

Why this matters

A user who sets BASE_CLI_TEMP_RETENTION_DAYS based on these docs, expecting it to control temp-directory pruning, would have it silently do nothing -- no error, no warning, just no effect. That's a worse failure mode than a missing doc entry: the user has no signal that anything is wrong.

Proposed change

Either:

• Remove BASE_CLI_TEMP_RETENTION_DAYS from both docs if it was never implemented or was removed without a doc update; or
• If temp-retention tuning was intended functionality, treat this as a missing-feature gap and implement it, updating the docs to match what actually ships.

Acceptance criteria

• docs/runtime-environment.md and docs/local-config.md only document environment variables that the code actually reads.