feat(config): add GOCLAW_CRON_JOB_TIMEOUT env var for cron timeout

[hoakhongmau98]

Summary

• Adds GOCLAW_CRON_JOB_TIMEOUT env var mapping in applyEnvOverrides(), completing the GOCLAW_* env var pattern for CronConfig.JobTimeout.
• Lets operators override cron job timeout without editing the JSON config file — useful for deployments running multi-turn LLM agents with large contexts where 10m default is too short.
• Zero breaking change: unset/invalid values fall through to the existing DefaultJobTimeout (10m) behavior via the existing JobTimeoutDuration() helper, which already handles parse errors with slog.Warn.

Closes #1098

Changes

internal/config/config_load.go — one new section in the operational-config block (next to Browser):

// Cron job execution
envStr("GOCLAW_CRON_JOB_TIMEOUT", &c.Cron.JobTimeout)

internal/config/config_load_test.go — 4 new test cases:

• TestLoad_CronJobTimeout_EnvVar — GOCLAW_CRON_JOB_TIMEOUT=1h → 1h
• TestLoad_CronJobTimeout_Invalid_FallsBackToDefault — invalid value → 10m (logs warn)
• TestLoad_CronJobTimeout_Unset_UsesDefault — unset → 10m
• TestLoad_CronJobTimeout_EnvOverridesFile — file 5m + env 30m → 30m (env wins)

Test plan

• go build ./... (PG build)
• go build -tags sqliteonly ./... (Desktop/SQLite build)
• go vet ./...
• go test ./internal/config/ — 4 new + all existing tests pass
• Manual: run gateway with GOCLAW_CRON_JOB_TIMEOUT=1h and confirm a long cron job is no longer killed at 10m

Notes for reviewers

• Placement chosen near the operational-config block (Sandbox/Browser) rather than the secrets block at the top, since cron timeout is a runtime parameter, not a credential.
• No new error handling needed — CronConfig.JobTimeoutDuration() already validates and falls back. This was confirmed by the existing TestCronConfig_JobTimeoutDuration_Invalid test.
• No changes to default behavior; existing deployments without the env var see no difference.