feat: make inbound envelopes configurable

Co-authored-by: Shiva Prasad <shiv19@users.noreply.github.com>

docs/concepts/timezone.md

@@ -9,7 +9,7 @@ read_when:
 
 Clawdbot standardizes timestamps so the model sees a **single reference time**.
 
-## Message envelopes (UTC)
+## Message envelopes (UTC by default)
 
 Inbound messages are wrapped in an envelope like:
 
@@ -17,7 +17,46 @@ Inbound messages are wrapped in an envelope like:
 [Provider ... 2026-01-05T21:26Z] message text
 ```
 
-The timestamp in the envelope is **always UTC**, with minutes precision.
+The timestamp in the envelope is **UTC by default**, with minutes precision.
+
+You can override this with:
+
+```json5
+{
+  agents: {
+    defaults: {
+      envelopeTimezone: "user", // "utc" | "local" | "user" | IANA timezone
+      envelopeTimestamp: "on", // "on" | "off"
+      envelopeElapsed: "on" // "on" | "off"
+    }
+  }
+}
+```
+
+- `envelopeTimezone: "user"` uses `agents.defaults.userTimezone` (falls back to host timezone).
+- Use an explicit IANA timezone (e.g., `"Europe/Vienna"`) for a fixed offset.
+- `envelopeTimestamp: "off"` removes absolute timestamps from envelope headers.
+- `envelopeElapsed: "off"` removes elapsed time suffixes (the `+2m` style).
+
+### Examples
+
+**UTC (default):**
+
+```
+[Signal Alice +1555 2026-01-18T05:19Z] hello
+```
+
+**Fixed timezone:**
+
+```
+[Signal Alice +1555 2026-01-18 06:19 GMT+1] hello
+```
+
+**Elapsed time:**
+
+```
+[Signal Alice +1555 +2m 2026-01-18T05:19Z] follow-up
+```
 
 ## Tool payloads (raw provider data + normalized fields)