feat: display inline images from tool results in chat buffer

[conornash]

When a tool (e.g. read) returns image content blocks alongside text, the images are now rendered inline inside the tool block overlay.

In GUI Emacs: decoded and displayed via create-image/insert-image with max-width/max-height scaling to fit the window.

In terminal Emacs: a text placeholder showing MIME type and size.

Implementation:

• New struct slot: tool-block.image-blocks stores content for toggle
• pi-coding-agent--mime-to-image-type: MIME string to Emacs type symbol
• pi-coding-agent--insert-inline-image: decode + insert or placeholder
• pi-coding-agent--insert-tool-images: iterate content, insert images
• display-tool-end: calls insert-tool-images after text rendering
• toggle-tool-output: re-inserts images from stored blocks after toggle

Handles: corrupt base64, unknown MIME types, empty data, vectors and lists, multiple image blocks, image-only results (no text).

12 new tests covering all paths. 942/942 full suite passes.

[dnouri]

Thanks @conornash for the original #221 work, and @bobrowadam for the #211 implementation consolidated here.

I’ve refreshed this PR onto current master and kept it focused on displaying image results:

• Completed tool-result images appear inline in graphical Emacs.
• Terminals show a useful image type-and-size placeholder.
• Raster images returned by Pi’s built-in read use Pi’s persisted image content; replay never reopens the original path.
• Complete, standalone SVG text returned by read can also be previewed. SVGs with obvious scripts or external resources remain ordinary text.
• Previews survive history replay, output toggling, and cooled tool history.
• Returned image sources are capped per image, and at most eight previews are shown per tool result.

The branch also keeps test dependencies project-local and passes the selected Emacs/package directory to child batch, GUI, and benchmark processes. This fixes the package lookup mismatch behind the old CI failures without changing normal compile behavior.

Verification is green: full make check, 18/18 headless GUI tests, native GUI coverage, fake integration, Emacs 29 focused coverage, empty-HOME package propagation, tool-update/reload smoke benchmarks, and a terminal smoke with PNG and SVG placeholders.

A few limits are intentional:

• Image blocks from partial tool updates are shown when the final result arrives.
• Pi’s built-in read resizes raster images, but unusual custom-tool images may still cost more decoder memory than their compressed size suggests.
• A chat first rendered in a terminal may need a toggle or history reload after moving to a GUI.
• SVG support is deliberately conservative rather than a general SVG sanitizer.

This remains display-only. Pasting an image or typing its path does not attach image data to an outgoing prompt; prompt image attachments remain separate #261 work.

[dnouri]

A final protocol-shaped review caught one small mismatch before merge: ordinary read results encode truncation as JSON null, and the SVG preview gate was treating Emacs’s :null sentinel as a real truncation record.

That is now corrected. Nil/JSON-null means the returned SVG is complete, while non-null truncation metadata still leaves it as text. Regression coverage exercises both cases, and all 11 replacement CI checks pass.