Implement target selection, autodiscovery via aggregator, implement listTargets

docs/USERGUIDE.md

@@ -324,6 +324,12 @@ endpoint for that request (useful for comparing a single collector against the a
 http://frontend:8080/?target=nginx3:9090&w=5m
 ```
 
+**Source picker** — when the frontend is pointed at an aggregator, a `source:` tab row appears
+below the dimension tabs listing each individual collector alongside an **all** tab (the default
+merged view). Clicking a collector tab switches the frontend to query that collector directly for
+the current request, letting you answer "which nginx machine is the busiest?" without leaving the
+dashboard. The picker is hidden when querying a collector directly (it has no sub-sources to list).
+
 ---
 
 ## CLI
@@ -338,6 +344,7 @@ Default output is a human-readable table; add `--json` for machine-readable NDJS
 logtail-cli topn    [flags]   ranked label → count table
 logtail-cli trend   [flags]   per-bucket time series
 logtail-cli stream  [flags]   live snapshot feed (runs until Ctrl-C)
+logtail-cli targets [flags]   list targets known to the queried endpoint
 ```
 
 ### Shared flags (all subcommands)
@@ -397,6 +404,32 @@ RANK  COUNT    LABEL
 {"ts":1773516180,"source":"col-1","target":"nginx1:9090","total_entries":823,"top_label":"example.com","top_count":10000}
 ```
 
+### `targets` subcommand
+
+Lists the targets (collectors) known to the queried endpoint. When querying an aggregator, returns
+all configured collectors with their display names and addresses. When querying a collector,
+returns the collector itself (address shown as `(self)`).
+
+```bash
+# List collectors behind the aggregator
+logtail-cli targets --target agg:9091
+
+# Machine-readable output
+logtail-cli targets --target agg:9091 --json
+```
+
+Table output example:
+```
+nginx1.prod                              nginx1:9090
+nginx2.prod                              nginx2:9090
+nginx3.prod                              (self)
+```
+
+JSON output (`--json`) — one object per target:
+```json
+{"query_target":"agg:9091","name":"nginx1.prod","addr":"nginx1:9090"}
+```
+
 ### Examples
 
 ```bash