docs: worker.local.env, and the search flag that silently does nothing

docs/WORKER_SETTINGS.md

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+# Worker settings (`worker.local.env`)
+
+Optional environment for the worker container that **survives a reinstall**.
+
+Before this existed, adding anything to the worker's environment meant editing the
+container by hand, and the next install from the app threw it away — the run
+command is generated fresh each time, so a manually-added `-e` flag disappeared
+with the old container. Nothing failed loudly; a capability simply stopped being
+advertised.
+
+## Where it goes
+
+```
+<keys-dir>/../worker.local.env
+```
+
+For a default install that is:
+
+```
+~/lightchain-worker/worker.local.env
+```
+
+The installer applies it with `docker run --env-file` when the file exists, and
+prints `▶ applying operator settings from …` so you can see it took effect.
+
+```bash
+chmod 600 ~/lightchain-worker/worker.local.env   # it will hold credentials
+```
+
+## Precedence
+
+This file is applied **before** the settings the installer manages
+(`RPC_URL`, `CHAIN_ID`, `WORKER_REGISTRY_ADDRESS`, `AI_CONFIG_ADDRESS`,
+`JOB_REGISTRY_ADDRESS`, `SUPPORTED_MODELS`, keystore paths, sortition config).
+
+That ordering is deliberate: those values win. A stale local file must not be able
+to repoint `RPC_URL` and leave a worker talking to the wrong chain while still
+looking healthy.
+
+Use it for capabilities and tuning, not for chain wiring.
+
+## Web search
+
+```ini
+SEARCH_ENABLED=true
+TAVILY_API_KEY=tvly-...
+SEARCH_MAX_RESULTS=6
+SEARCH_TIMEOUT=30s
+```
+
+> ### ⚠️ The variable is `SEARCH_ENABLED`, with a **D**
+>
+> `SEARCH_ENABLE` (no D) is read by nothing. Set it and the worker starts
+> cleanly, reports healthy, and **never advertises search** — a silent failure
+> with no error to notice and no log line to grep for.
+>
+> Verify against whatever image you are running rather than trusting this file:
+>
+> ```bash
+> docker exec lightchain-worker sh -c \
+>   "strings /bin/worker | grep -oE '\bSEARCH_[A-Z_]+\b'" | sort -u
+> ```
+>
+> At the time of writing that returns exactly `SEARCH_ENABLED`, and
+> `SEARCH_ENABLE` does not appear in the binary at all.
+
+## Applying a change
+
+The env of a running container cannot be altered, so it has to be recreated.
+Re-running install from the app is the supported path and will pick the file up.
+
+## Verifying
+
+```bash
+docker exec lightchain-worker env | grep -E '^SEARCH_|^TAVILY'
+docker inspect lightchain-worker --format '{{.RestartCount}}'   # want 0
+```
+
+A non-zero and climbing restart count means the worker is crash-looping — check
+`docker logs lightchain-worker`. A malformed line in this file (no `=`) will make
+`docker run` reject the whole `--env-file`.
+
+## Two things that will bite
+
+**Run as root.** The installer starts the worker with `--user root`, and
+`session-keys.enc` is written `root:root 0600`. Recreating the container without
+`--user root` gives
+`open /data/session-keys.enc: permission denied` in a restart loop.
+
+**Chain settings are not optional.** This file alone is not a complete
+environment. Starting a container with only `--env-file worker.local.env` fails
+with `CHAIN_ID is required` and four siblings — the installer's `-e` flags supply
+those.