feat: add multi-attachment delivery batching

docs/05-channels-messaging.md

@@ -105,6 +105,24 @@ Splitting is intentionally conservative. Replies containing fenced code, tables,
 
 Progress messages are not added to session history by this behavior. Existing run timeline handling for explicit `block.reply` remains unchanged.
 
+### Outbound Media Delivery
+
+Agent tools can queue media files through `Result.Media`; the consumer converts
+those entries into `OutboundMessage.Media` and preserves file order, MIME type,
+filename, and optional captions.
+
+- `send_file` accepts either the legacy single `path` + optional `caption`, or
+  `attachments: [{path, caption?}, ...]` for batch delivery of existing
+  workspace files.
+- Telegram groups compatible outbound media into `sendMediaGroup` album chunks
+  of 2-10 items. Photo/video items can share a chunk; documents group only with
+  documents; audio groups only with audio. Voice-mode audio, singleton chunks,
+  oversized images sent as documents, and incompatible runs use the existing
+  ordered single-send fallback.
+- Discord already sends multiple files plus optional text in one message.
+- Slack and other media-capable channels keep ordered fallback behavior unless
+  their adapter advertises a stronger batch capability.
+
 ### Reasoning Delivery
 
 Telegram channel config supports explicit reasoning delivery:

docs/project-changelog.md

@@ -6,6 +6,27 @@ Significant changes, features, and fixes in reverse chronological order.
 
 ## 2026-06-12
 
+### Multi-attachment outbound delivery (issue #172)
+
+**Changes**
+
+- Extended `send_file` with `attachments[]` so agents can queue multiple
+  existing workspace files in one tool call while preserving order and captions.
+- Added caption propagation from tool result media through agent media results
+  into outbound channel attachments.
+- Added Telegram media-group delivery for compatible batches with 2-10 item
+  chunks, plus ordered fallback for singleton, voice, oversized-image, or mixed
+  incompatible cases.
+- Added explicit channel batch capability metadata for Telegram, Discord, and
+  ordered fallback channels.
+
+**Tests**
+
+- Added batch `send_file` regressions for happy path, duplicate rejection, and
+  all-or-nothing delivered-state handling.
+- Added Telegram media-group chunking tests for compatible grouping, max-size
+  chunking, and non-groupable fallback.
+
 ### Operator trace CLI (issue #158)
 
 **Changes**