Improve Kit startup context and actionable CLI errors

[Copilot]

Kit startup output did not tell users or agents what Kit is or which config file was actually loaded, and top-level failures surfaced as raw errors without clear recovery guidance. This change makes startup output self-describing and normal user errors explicit about what failed, why, and what to do next.

• Startup context

  • Emit a stable startup line describing Kit and including build/version when available.
  • Emit the resolved config file path on startup.
  • Distinguish config source as default vs explicit.

• Top-level error shaping

  • Route CLI execution through a single testable entrypoint.
  • Normalize user-facing failures into three lines:
    • kit: error: ...
    • kit: cause: ...
    • kit: next: ...

• Covered failure classes

  • Invalid CLI flags / argument parsing
  • Invalid or inaccessible working directory
  • Config read / parse / rewrite failures
  • Workflow selection / validation / task failure summaries

• Behavior notes

  • Keep existing non-zero exit behavior for failures.
  • Preserve help/version/completion flows while improving completion error wording.

• Tests

  • Add focused coverage for startup messaging, config source/path reporting, malformed config handling, invalid flags, and runtime task failures.

Example of the new surface:

kit: startup: workflow engine for software development; version=(devel)
kit: config: path=/workspace/tasks.yaml source=default

kit: error: workflow run failed: failed tasks: [job]
kit: cause: one or more tasks exited with a non-zero status
kit: next: inspect the task output above or the logs/ directory and retry

[Copilot]

Pull request overview

This PR refactors Kit’s CLI entrypoint to emit clearer startup context (what Kit is, version when available, resolved config path + source) and to normalize top-level user-facing failures into consistent kit: error/cause/next messaging, with focused tests to validate the new output surfaces.

Changes:

• Refactor main into a single testable run/execute flow and add stable startup + config-source/path lines.
• Add structured, actionable error shaping for flag parsing, working directory failures, config read/parse/rewrite, and common runtime workflow failures.
• Add unit tests asserting startup messaging and actionable error formatting for several failure classes.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 4 comments.

File	Description
main.go	Introduces run/execute, prints startup/config context, and shapes common CLI/runtime errors into actionable 3-line messages.
main_test.go	Adds coverage for startup/config reporting and for the new normalized error surfaces.

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