Attach images to outgoing prompts

[dnouri]

Summary

Add a narrow, explicit path for outgoing image prompts:

• Press C-c C-a in the input buffer and select—or paste—the path to one PNG, JPEG, GIF, or WebP image. C-u C-c C-a clears it.
• Emacs identifies the format from the file bytes, materializes the image immediately, and keeps its basename and size visible in the input header.
• A nonempty ordinary prompt sends the image through Pi's documented images RPC field when the selected model advertises image input.
• Accepted user images reuse the bounded image renderer from feat: display inline images from tool results in chat buffer #221 in live chat and history, with useful type-and-size placeholders in terminals.
• Prompt refusal, RPC failure, model-selection failure, process replacement, and reload preserve the draft or restore queued text rather than losing it or sending it under the wrong model.
• Process-bound model-change tokens prevent stale callbacks from changing a replacement session.
• Extension-handled prompts retract speculative image turns only after Pi authoritatively reports idle; delayed real turns remain owned until agent_start arrives.
• The fake Pi contract persists image blocks, providing deterministic subprocess-level coverage.

Image-bearing drafts remain deliberately outside local follow-up and steering queues. Busy, slash-command, steering, unresolved-capability, and text-only-model attempts refuse without clearing the draft.

Stack

This PR is stacked on #273, which extracts the shared text-prompt settlement and session-reset ownership fixes found during review. This branch contains the image feature plus image/model-specific integration and hardening. Once #273 merges, this PR can be retargeted to master without changing its feature diff.

Intentional limits

• One attached image per prompt.
• Direct idle prompts with nonempty text only.
• Images are sent as-is under a configurable 3 MiB source limit; there is no resize or conversion step.
• Clipboard image extraction, automatic path detection in prompt prose, multiple images, and image-bearing queue/steering turns remain separate work.
• Emacs file handlers such as TRAMP may supply readable paths, but remote attachment reads have not been broadly exercised.
• Pi extensions can change the active model without an authoritative RPC model-change event; a later extension replacement may require a history reload to update the live preview.

Verification

• make check on the combined stack — 1,680 tests, one compatibility skip, no unexpected results
• make test-integration-fake — all 15 fake contracts passed
• Real Emacs -Q terminal and Xvfb GUI image-send/render probes passed
• Delayed agent_start, extension-handled prompt, model/process replacement, reload, and reset race probes passed
• Python compile, Ruff lint/format, checkdoc, package-lint, byte compilation, and git diff --check passed

Closes #261.