fix: resolve structured action context inputs

[Optic00]

Summary

• Centralize Action execution-context lookup across templates, ai_extract, and ai_agent.
• Resolve editor-provided item.* inputs, nested output paths, slices, trigger data, prior values, users, and SCM payloads.
• Preserve tolerant template placeholders while making missing configured AI inputs fail with a clear error.
• Serialize structured template and extraction values as JSON.

Behavior note

ai_agent previously skipped unresolved configured inputs and could run with a partial or empty prompt. It now fails the node before starting an LLM call. Present null values remain valid inputs.

Tests

• go test ./internal/services
• go test ./... after satisfying the required frontend/dist embed input

This PR includes focused public unit coverage. Test placement is also being discussed in #193; I am happy to adapt it to the maintainers preferred convention.