feat: add symphony workflow preview command

[chihsuan]

Context

Authors editing WORKFLOW.md had no way to see the final prompt an agent receives; partials and variables were only resolved at runtime, so template mistakes surfaced mid-run.

TL;DR

Add symphony workflow preview to print the assembled base-issue prompt from a local WORKFLOW.md.

Summary

• Add symphony workflow preview [--file WORKFLOW.md] [--agent codex|claude] escript subcommand.
• Render the exact base-issue prompt with deterministic sample data — no Linear, network, or orchestrator.
• Reuse PromptBuilder.build_prompt/2 via a new optional :workflow opt so preview can't drift from runtime.
• Add SymphonyElixir.WorkflowPreview (render/1, sample_issue/0); surface bad partials/vars as errors (lint).
• Document the command in README.md; add a non-rendering front-matter tip in WORKFLOW.md.

Alternatives

• Static partial-expansion only (variables left as placeholders): rejected — does not show what the agent actually receives.
• Fetch a real Linear issue for variable values: rejected — needs auth/network and is non-deterministic; sample data is enough to verify structure.
• Mix task instead of escript subcommand: rejected — end users run the shipped binary, not the source checkout.

Test Plan

• make all
• mix test test/symphony_elixir/workflow_preview_test.exs test/symphony_elixir/cli_test.exs test/symphony_elixir/prompt_builder_test.exs
• ./bin/symphony workflow preview renders the repo WORKFLOW.md; unknown {% render %} partial reports an error

[Copilot]

Pull request overview

Adds a new CLI subcommand to preview the fully assembled base-issue agent prompt from a local WORKFLOW.md, using deterministic sample issue data and the same prompt-building path as runtime to avoid drift.

Changes:

• Add symphony workflow preview CLI subcommand to print the rendered base-issue prompt (with sample issue context) without needing Linear/network/orchestrator.
• Introduce SymphonyElixir.WorkflowPreview and a :workflow seam in PromptBuilder.build_prompt/2 to reuse the runtime prompt assembly code path.
• Add tests for preview behavior and update docs (README.md, WORKFLOW.md) to advertise the command.

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
WORKFLOW.md	Adds a front-matter tip about the new preview command without affecting rendered prompt content.
README.md	Documents symphony workflow preview usage and intent as a lint/preview tool.
lib/symphony_elixir/workflow_preview.ex	New module that loads a workflow file and renders the assembled prompt with sample issue data.
lib/symphony_elixir/prompt_builder.ex	Adds :workflow opt handling so preview can reuse the same prompt assembly logic as runtime.
lib/symphony_elixir/cli.ex	Adds workflow preview command wiring and usage text.
test/symphony_elixir/workflow_preview_test.exs	New unit tests for preview rendering and error handling.
test/symphony_elixir/prompt_builder_test.exs	Verifies PromptBuilder.build_prompt/2 supports pre-loaded workflows via :workflow.
test/symphony_elixir/cli_test.exs	Adds CLI-level tests for preview output and usage/error behavior.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[chihsuan]

Fixed in 2d5a8e5. resolved_workflow/2 now matches {:error, _reason} and propagates it, so a Workflow.load/1 error flows to prompt_template!/2 and raises the existing workflow_unavailable: ... error instead of a CaseClauseError. Added tests covering the {:ok, workflow} and {:error, reason} shapes.

[chihsuan]

Fixed in 2d5a8e5. load_error_message/2 now uses :file.format_error/1 for the POSIX reason, so :enoent and :eacces read differently ("no such file or directory" vs "permission denied"), and adds a dedicated {:workflow_parse_error, _} clause that reports a readable "Could not parse front matter" message. The generic fallback is clearly labeled "Could not load workflow file". Added tests for missing-file, malformed-YAML, and non-map front matter.