feat(telegram): send and receive JPEG/PNG images

[hegelstad]

Summary

Add image support to the existing Telegram bridge, without a separate service or harness-specific prompt protocol.

• Receive Telegram photos and JPEG/PNG documents. Save validated image bytes in a private $H2_DIR/attachments/<bridge-name>/ directory, then deliver the caption and local image path through the existing agent-message route.
• Add h2 send <bridge> --image ./image.png [caption], including --closes support. Image uploads use an optional bridge.ImageSender capability; text-only bridges are unchanged.
• Preserve caption prefixes and reply-to-agent routing. Image captions are not executed as slash commands.
• Use a distinct send-image socket operation so an older bridge rejects the request instead of silently discarding the image.

Safety and scope

• Configured-chat authorization before downloading, bounded metadata/downloads, JPEG/PNG decoding and dimension checks, private files with generated names, no remote filenames used locally.
• Image HTTP requests have deadlines, reject redirects, and omit token-bearing URLs/remote response bodies from errors. Error notifications respect cancellation.
• Upload only explicitly selected regular local files; do not interpret paths or URLs in message text as upload instructions.
• Inbound images are local file references, not native multimodal prompt injection. Agents need an image-viewing tool and access to the attachment directory.
• One image per send; photo albums arrive as separate Telegram updates. No GIF/sticker/video support, automatic retention cleanup, or automatic upload retry in this change. Those limitations are documented.
• No production daemon restart or live bot credentials were used for validation.

Verification

• make check — pass (gofmt, go vet, staticcheck).
• make test — pass.
• make test-external — pass with inherited H2_* session variables removed and temporary h2 roots. An initial run inherited H2_POD=ops, which filtered pod-list fixtures; rerunning in the isolated environment passed.
• go test -race ./internal/bridge/telegram ./internal/bridgeservice -count=1 — pass.
• make build — pass.

New tests cover multipart upload, captionless sends and --closes, routing, file validation, chat authorization, size/path/redirect errors, error redaction, and socket capability handling. A synthetic-image integration test goes through Bot API polling → local attachment → agent socket → bridge socket → Bot API upload. The HTTP API and agent are test doubles; the Telegram implementation and bridge service are real.

The broader CLI race build was stopped while instrumenting the large treesitter Bash grammar to avoid exhausting the development host's memory; the CLI is covered by the normal full and external suites, while the transport packages passed race testing.

Usage and retention/privacy details: docs/telegram-images.md.

[hegelstad]

Any interest for image sending and receiving support? I have found it very useful to send screenshots back to the concierge to show how something looks or how I want it to look.