feat(backend): add extract-text node for PDF/DOCX documents

[LukasHirt]

Summary

Implements the "Extract Text" node (previously just a design proposal in docs/superpowers/specs/2026-07-24-new-workflow-node-types-design.md) end to end.

Bug this fixes: Executor.Run (backend/pkg/executor/executor.go) has always done vars["file.content"] = string(content) unconditionally for every file type. That's correct for .txt/.md, but for a PDF or .docx, an LLM Prompt node referencing {{file.content}} silently got that binary format's raw bytes stringified instead of the document's actual text — a real, user-facing gap for a perfectly reasonable "summarize this uploaded PDF" workflow.

What's in scope: plain-document text extraction (PDF and DOCX). OCR is explicitly out of scope — recovering text from an image (a scanned PDF, a photo of a page) needs either a bundled OCR engine or a call out to an external service, a meaningfully bigger lift than parsing an already-textual document format. That remains a natural, separate follow-up; nothing here attempts it.

Backend (backend/pkg/textextract, backend/pkg/executor)

• New textextract package: DetectFormat (by extension, falling back to content-sniffing magic bytes/zip structure) and Extract, which converts PDF/DOCX to plain text and passes through anything else (including plain text) unchanged — no behavior change for existing .txt/.md workflows.
• PDF parsing uses github.com/ledongthuc/pdf — pure Go, BSD-3-Clause (a maintained fork of the Go team's own rsc.io/pdf), no cgo, license compatible with this project's existing dependencies. License text and NOTICE.md/REUSE.toml entries added; reuse lint passes.
• DOCX parsing uses only the standard library (archive/zip + encoding/xml) — a .docx is just a zip with an XML part, and plain-text extraction needs nothing beyond unzip-and-walk-the-XML, so no new dependency was needed for that half.
• New executor node kind extractText (runExtractText, parallel to runLLM/runAction): reads the same file.content/file.name vars Run already loads, extracts text, and writes it to vars["file.text"] by default or node.Data["outputVariable"] when set. result.Output gets a short "N characters extracted" summary (per spec, not the full text — unlike llm.output, which can be arbitrarily large here).

Frontend

• Modeled as a new CanvasNodeKind ('extractText'), not a new actionType. Rationale: it has no "target file" side effect the way tag/move/comment/etc. do — it's a content-transform step, like the LLM Prompt node. The LLM node is already precedent in this codebase for a single-purpose top-level kind with no sub-types, so this keeps the picker/canvas/NDV wiring consistent with how llm already works instead of overloading the action-node switch.
• New AI-category picker entry (file-text icon), a new canvas node component (ExtractTextNode.vue, mirroring LlmNode.vue), and the optional output-variable-override field wired into NodeDetailsPanel.vue.
• Reuses the existing outputVariable field already declared (but unused) on WorkflowNodeData rather than adding a new field — same name, same shape, for consistency. (The LLM node's own NDV doesn't read it yet — this PR doesn't touch that, only extractText does.)

Test plan

• Backend: TDD — wrote pkg/textextract tests first (confirmed they failed to compile with no implementation), then implemented DetectFormat/Extract/PDF/DOCX extraction. Tests build minimal-but-valid PDF/DOCX fixtures in-process (real byte-accurate PDF with xref table; real DOCX zip with word/document.xml) rather than checking in binary fixtures.
• Backend: extended pkg/executor/executor_test.go with runExtractText tests — default output var, custom output var, real end-to-end PDF extraction through the executor, plain-text passthrough, and failure propagation on a corrupt document (execution stops, downstream llm node doesn't run).
• cd backend && go build ./... && go vet ./... && go test ./... — all green.
• Frontend: added tests/unit/nodeTypes.spec.ts (Extract Text registered in NODE_TYPES under AI_CATEGORY) and tests/unit/NodeDetailsPanel.spec.ts (output-variable field renders and emits correctly), confirmed both fail before implementation, pass after.
• pnpm test:unit, pnpm check:types, pnpm lint, pnpm build — all green.
• reuse lint — compliant (added LICENSES/BSD-3-Clause.txt, NOTICE.md/REUSE.toml entries for ledongthuc/pdf).

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