Fix __GH_AW_WIKI_NOTE__ placeholder not substituted when wiki is disabled (#20236)

* Initial plan

* Fix __GH_AW_WIKI_NOTE__ placeholder not substituted when wiki is disabled

When wiki mode is disabled (memory.Wiki == false), the GH_AW_WIKI_NOTE
env var now uses ${{ '' }} (a GitHub expression evaluating to empty
string) instead of a plain static empty string.

This forces expression interpolation, ensuring the __GH_AW_WIKI_NOTE__
placeholder in repo_memory_prompt.md is always substituted with an empty
value. Previously, old compiled lock files might not include GH_AW_WIKI_NOTE
in the substitution step at all, causing the validate_prompt_placeholders.sh
check to fail.

Fixes: repo-assist __GH_AW_WIKI_NOTE__ placeholder not substituted when
Wiki is disabled.

Recompile all 166 workflow lock files to apply the fix.

Co-authored-by: pelikhan <4175913+pelikhan@users.noreply.github.com>

* Apply PR review suggestions: add constant and clarify comment

- Add package-level constant `ghaEmptyStringExpr` for `${{ '' }}` and
  use it in both the production code and the test.
- Reword the comment to clarify the expression ensures substitution in
  newly compiled (or regenerated) workflows by forcing the prompt-
  creation step to include GH_AW_WIKI_NOTE.

Co-authored-by: pelikhan <4175913+pelikhan@users.noreply.github.com>

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: pelikhan <4175913+pelikhan@users.noreply.github.com>
Co-authored-by: Peli de Halleux <pelikhan@users.noreply.github.com>

Author: Copilot