ci(dispatch): keep run artifacts out of the commit

[mike-diff]

Every dispatch PR commits a TASK.md to the repo root. #41 is the current
example: a 46-line TASK.md sits alongside the actual fix.

Cause

The commit step strips one artifact but not the other:

rm -f PR_BODY.md      # stripped, with a comment explaining why
git add -A            # ...but TASK.md, written earlier in the same run, is swept in

TASK.md is written by the "Build the task file" step and never removed, so
git add -A picks it up. The reasoning already in the workflow for stripping
PR_BODY.md applies word for word: it describes the change but is not a source
change. TASK.md is even further from source: it is the prompt the run was
given.

Change

Two layers, deliberately:

• rm -f PR_BODY.md TASK.md in the commit step, so the tree is clean.
• /TASK.md and /PR_BODY.md in .gitignore, so a missed delete cannot land
  them anyway.

The .gitignore entries are the guarantee and the rm is the tidy-up. An
unattended workflow should not depend on remembering a delete: the declarative
rule holds even if the imperative one is edited away later. Both are anchored
with a leading / so only the repo root is affected, never a TASK.md that
belongs to something else deeper in the tree.

Verification

• dispatch.yml parses, and the strip is asserted present in the
  Commit, push, open PR step.
• git check-ignore -v confirms the rules bind:

TASK.md          ignored=True   .gitignore:11:/TASK.md
PR_BODY.md       ignored=True   .gitignore:12:/PR_BODY.md
harness/tui.go   ignored=False

Scope

.github/workflows/dispatch.yml and .gitignore. No Go code touched.

Does not retroactively clean #41; that file is removed in that PR directly.