fix(signal): inline image and PDF attachments as base64

[brentkearney]

Fixes #2528.

Type of Change

• Fix — image attachments unreachable from the agent container
• Feat — adds PDF attachment handling (previously dropped silently)

Problem

src/channels/signal.ts was emitting {path: <host-path>} for image attachments in the structured attachments array, plus a [Image: <host-path>] line in the text body. The host never mounts signal-cli's attachments directory (~/.local/share/signal-cli/attachments) into agent containers, so the agent received a path that didn't resolve inside the container. Vision-capable models couldn't see images; PDFs were dropped entirely (no handler for application/pdf).

Full root cause and reproduction in the linked issue.

Solution

The host already has a complete pipeline: session-manager.extractAttachmentFiles saves any attachments[].data base64 to <sessionDir>/inbox/<msgId>/<filename> (= /workspace/inbox/... in container) and rewrites the entry as localPath. The container formatter renders [image|document: <name> — saved to /workspace/inbox/.../...].

signal.ts just needs to read each attachment off disk and inline it as base64 in the attachments array. No host changes, no new mount, no new dependency.

Changes

• src/channels/signal.ts: extract a shared inlineAttachment helper, call it once for image/* attachments and once for application/pdf attachments. Helper reads file, base64-encodes, pushes {data, name, type, contentType, size}. Preserves the original filename when signal-cli supplies one (typical for documents); synthesizes <id>.<ext> otherwise. Drop the now-redundant [Image: <host-path>] line — the formatter renders its own ref line from the structured attachment.
• src/channels/signal.test.ts: existing image test updated to assert the new structured shape (data, name: '<id>.jpeg', type: 'image') and an empty text body. New PDF test covers filename preservation and type: 'document'. Both tests stage a real file in tmpdir and round-trip its base64 through the adapter. 38/38 tests pass.

Voice attachments

Unaffected. They're transcribed on the host (via WHISPER_BIN / OPENAI_API_KEY) before being embedded in the text content, so no file is passed to the container.

Verified

Running live on a downstream install since 2026-05-17. Confirmed end-to-end:

• Send an image on Signal → agent describes it correctly.
• Send a PDF on Signal → agent reads its contents.

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