… site (coleam00#1506)

* fix(simplify): stage only edited files, forbid scratch artifacts

The simplify command used `git add -A`, which sweeps untracked review/
report files (e.g. `review/scope.md` left by upstream review nodes) into
the simplification commit. Replace it with explicit per-file staging
using the list of paths edited in Phase 2, plus a forbidden-paths list
so review artifacts, PR-body scratch files, and anything under
`$ARTIFACTS_DIR` cannot leak into the commit.

* fix(fix-github-issue): forbid scratch artifacts in create-pr step

The inline create-pr prompt told the agent to "stage and commit" any
uncommitted changes, which lets transient artifacts from upstream nodes
(`.pr-body.md`, `review/scope.md`, scratch reports) land in the
implementation commit and PR diff. Replace the loose instruction with
explicit per-file staging, a forbidden-paths list, and a rule that any
PR body file written for `--body-file` must live at
`$ARTIFACTS_DIR/pr-body.md` or `/tmp/` — never inside the worktree.
Applied to both the default and experimental variants.

* fix(workflows): purge remaining git add -A in worktree-context steps

Same class of bug as the simplify and create-pr fixes: every
worktree-facing default that used `git add -A` could sweep transient
review/scratch artifacts (`.pr-body.md`, `review/scope.md`,
`*-report.md`, anything left under `$ARTIFACTS_DIR`) into the commit.
Replace with explicit per-file staging plus a forbidden-paths list and
a `git status --porcelain` verification step.

Touched:
- commands: archon-create-pr, archon-finalize-pr, archon-fix-issue,
  archon-implement-issue, archon-implement-review-fixes
- workflows: archon-piv-loop (3 sites), archon-ralph-dag,
  archon-refactor-safely

Intentionally left as `git add -A`:
- archon-release.yaml: working tree validated clean before this step;
  comment already explains why.
- archon-adversarial-dev.yaml: operates inside `$ARTIFACTS_DIR/app/`,
  a dedicated scratch repo, not the user's worktree.