Skills must be idempotent on stage re-run

## Context

From [#56 discussion](https://github.com/konveyor/agentic-controller/issues/56#issuecomment-5134307097).

Re-running a stage re-clones and re-executes from scratch. If a skill (e.g. plan) has already produced its artifact (e.g. `spec.md`) and the user approved it, a retry will silently overwrite the approved artifact.

## Problem

Each skill needs to check whether its own artifact already exists on the branch before generating a new one. Without this, an approved `spec.md` can be silently replaced by a re-run, losing the user's approval and any feedback they incorporated.

## Expected behavior

- If the artifact already exists and was approved, the skill should skip generation and proceed to the next phase (e.g. `implementation.md` generation).
- If the artifact exists but was not approved (no decision file), the skill should either resume the approval flow or regenerate.
- The skill should log clearly whether it is generating fresh or resuming from an existing artifact.

## References

- Parent: #56
- Related: #54

No specific files or tests are named. Start by tracing the stage re-run path into skill execution, then inspect how artifacts and decision files are checked. Done means approved artifacts are preserved, unapproved artifacts resume or regenerate as defined, and logs distinguish fresh generation from resumption.

Written by the indexing model from the issue text.