#12012: Gateway/Plugins: skip auto-enable config write in Nix mode

## Summary - Skip `writeConfigFile` in the plugin auto-enable code path when `OPENCLAW_NIX_MODE=1` is set - Log detected auto-enable changes as info without persisting to disk - Consistent with the existing `isNixMode` guard on legacy config migration ## Problem When the gateway starts in Nix mode, the config file (`openclaw.json`) is a read-only symlink to the Nix store. The `applyPluginAutoEnable` code path calls `writeConfigFile()` unconditionally, which: 1. Tries an atomic rename (fails — read-only nix store) 2. Falls back to `copyFile()`, replacing the symlink with a regular file 3. Subsequent `home-manager switch` fails with "Existing file would be clobbered" The legacy migration path already checks `isNixMode` and throws instead of writing, but the plugin auto-enable path was missing this guard. ## Fix When `isNixMode` is true, skip the `writeConfigFile` call and log the changes as informational only. In Nix mode, plugins should be declared in the Nix config rather than auto-enabled at runtime. Related: openclaw/nix-openclaw#50, openclaw/nix-openclaw#51 ## Test plan - [ ] Start gateway with `OPENCLAW_NIX_MODE=1` and a config that triggers plugin auto-enable (e.g. telegram channel configured) - [ ] Verify config symlink is preserved (not replaced with regular file) - [ ] Verify log shows "skipping plugin auto-enable persistence (Nix mode)" message - [ ] Verify normal (non-Nix) mode still writes config as before 🤖 Generated with [Claude Code](https://claude.com/claude-code) <!-- greptile_comment --> <h2>Greptile Overview</h2> <h3>Greptile Summary</h3> This change updates gateway startup so that when `OPENCLAW_NIX_MODE=1` is detected (`isNixMode`), plugin auto-enable changes are **logged but not persisted** to `openclaw.json`. In normal (non-Nix) mode the previous behavior remains: auto-enable updates are written via `writeConfigFile()` and then logged. This fits alongside the existing Nix-mode behavior earlier in `startGatewayServer()` where legacy config migration is blocked from writing to disk. The net effect is to avoid clobbering a read-only/symlinked config file in Nix-managed setups while keeping auto-enable persistence for standard installs. <h3>Confidence Score: 5/5</h3> - This PR is safe to merge with minimal risk. - The change is narrowly scoped to a single startup code path and only alters persistence behavior behind an existing `isNixMode` guard. The non-Nix path preserves prior write-and-log semantics, and the Nix path avoids an expected failure mode (attempting to write to an immutable/symlinked config). No other code paths are affected. - No files require special attention <!-- greptile_other_comments_section --> _{(2/5) Greptile learns from your feedback when you react with thumbs up/down!} <!-- /greptile_comment -->