feat(app): fall back to text part for non-multimodal attachments

[yashranaway]

Refs #1777

Summary

• Branch composer attachments by MIME instead of unconditionally encoding every file as a base64 type: "file" data URL. Images and PDFs keep the existing multimodal path. Text-like files (text/* plus a small allowlist of application/{json,xml,yaml,csv,javascript,typescript,sql,x-sh,x-yaml}) are emitted as type: "text" parts with the content inlined between [attached file: name] / [end of name] markers so the agent can reason about them without provider-side rejection.
• Cap inline text at 256 KB and emit a clear "too large to inline, save it to the workspace" notice above that size so very large CSVs do not balloon the prompt.
• For binary attachments outside the multimodal set (xlsx, zip, etc.), emit a placeholder text part naming the file and asking the agent to read it from the workspace via Read/Bash, instead of silently sending a payload the provider will drop.
• Slash commands still accept file parts only (SDK constraint), so the new text-fallback parts are filtered out of buildCommandFileParts rather than passed through as an invalid shape.

Verification

• pnpm --filter @openwork/app typecheck reports the same pre-existing origin/dev errors as before (missing @heroicons/react/24/solid, xlsx, marked-emoji, marked-shiki, emojilib, @shikijs/transformers, shiki type decls); no new errors land in actions-store.ts.
• node scripts/i18n-audit.mjs --ci passes — no i18n surface changed.

Notes

• The reporter on Feature request: native path-injection convention for non-multimodal attachments in the web composer #1777 raised the broader design question of writing attachments to a real workspace path so MCP/Bash can reach them via tools. That is the right long-term shape but needs the Electron bridge plumbing and per-platform tempdir lifecycle. This PR is the narrower fix that unblocks "agent reasons about a CSV/JSON/code attachment" today without provider-side drops, and leaves the path-on-disk story for a follow-up that pairs with the desktop write surface.
• The MIME allowlist is intentionally small; happy to grow it once we see common attachment types in user reports.

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Project	Deployment	Actions	Updated (UTC)
openwork-landing	Ready	Preview, Comment, Open in v0	May 22, 2026 4:22pm

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