docs(add-signal-v2): document XDG attachment path behaviour and voice fix

signal-sdk launches signal-cli without --config, so attachments land at
~/.local/share/signal-cli/attachments/ (XDG default) rather than
data/signal/. Document this in the Channel Info section and add a
troubleshooting entry explaining the symptom (voice messages silently
skipped, no transcript), how to confirm (ps aux | grep signal-cli), and
the automatic fallback the adapter uses.

.claude/skills/add-signal-v2/SKILL.md

@@ -297,6 +297,11 @@ ls data/models/ggml-*.bin 2>/dev/null || echo "no model — download one"
   as shown above.
 - **config-lock**: signal-sdk holds an exclusive lock on `data/signal/` while
   nanoclaw is running. Stop the service before running any `signal-cli` commands.
+- **attachment storage**: signal-sdk launches signal-cli **without** a `--config`
+  flag, so signal-cli stores attachments at the XDG default
+  (`~/.local/share/signal-cli/attachments/`), not under `data/signal/`. The
+  adapter checks both locations. Verify with:
+  `ps aux | grep signal-cli` — if there is no `-c` argument, XDG default is in use.
 
 ---
 
@@ -327,6 +332,15 @@ GroupV2. The adapter must extract the group ID from
 GroupV1/legacy). Check `src/channels/signal.ts` and confirm the groupId
 extraction falls through to `groupV2.id`.
 
+**Voice messages / attachments silently skipped, no transcript** — signal-sdk
+launches signal-cli without a `--config` flag, so attachments land at the XDG
+default (`~/.local/share/signal-cli/attachments/`) rather than under
+`data/signal/`. Confirm with `ps aux | grep signal-cli` — if there is no `-c`
+argument in the process line, the XDG default is in use. The adapter falls back
+to that location automatically. If you still see no "Signal attachment saved"
+log lines, add a debug log around the `if (!storedPath) continue` guard in
+`src/channels/signal.ts` to inspect `att.storedFilename` and `att.id`.
+
 **Java not found** — install Java 17+ (see Install step 1).
 
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