feat: recover from image rejections and add Files API image upload

[mbuckaway]

Related Issue

Not tied to an upstream issue — fork-internal change requested by the maintainer.

Problem

When a provider (e.g. DeepSeek) rejects an image with a 400, the session is permanently frozen: the offending image stays in the conversation history and is re-sent on every subsequent prompt, so every turn fails until the user starts a new session. There is also no way to send an image via the provider's Files API — only inline base64 and passthrough URLs — so large images or images over the 48 MiB body limit cannot be processed.

What changed

• Classify DeepSeek's image-rejection 400 messages as recoverable image errors in both engines (agent-core-v2 and the v1 kosong/agent-core).
• Make the media-strip sticky across turns and durable across a session reload, so a rejected image stays stripped instead of being re-sent.
• Capability-gate remote http(s) media (previously only daemon-file media was gated) and drop the non-standard id field on the OpenAI image_url wire part.
• Add a config-declared image_file_api capability that sends images via the provider's Files API (file content part with file_id), alongside inline base64 and URL passthrough.

Checklist

• I have read the CONTRIBUTING document.
• I have linked a related issue (fork-internal change; no upstream issue).
• I have added tests that prove the feature works.
• Ran gen-changesets skill.
• Ran gen-docs skill, or this PR needs no doc update.