Fix Repo Assist memory migration

[dsyme]

Summary

• move legacy Repo Assist memory cleanup into a deterministic pre-agent step
• remove every stale memory entry except notes.json, including legacy runid.json and non-JSON files
• keep custom memory validation read-only and restore max-file-count to one
• report the actual unexpected filenames when validation fails

This fixes upgraded repositories whose existing memory/repo-assist branch contains files from older Repo Assist memory formats.

Validation

• gh aw compile workflows/repo-assist.md --approve
• gh aw compile --validate workflows/repo-assist.md
• exercised migration against legacy files and a legacy directory; only notes.json remained

Security review

The compilation reported a changed memory-validation script and required explicit approval. I reviewed the change: it only removes the validator's filesystem mutations and improves its error message. No secrets, actions, or redirects were added or removed. The new migration step operates only within /tmp/gh-aw/repo-memory/default and preserves notes.json.

[dsyme]

Compiled and traced this change in fslaborg/Deedle with gh-aw v0.89.21.

The generated order is correct:

1. clone memory/repo-assist
2. run the migration before the agent
3. filter and validate memory after the agent
4. upload the artifact
5. validate and push it in push_repo_memory

For Deedle this now works because its memory branch has already been manually cleaned to contain only notes.json.

However, this PR alone does not migrate a still-dirty remote memory branch. The agent job removes legacy files from its working copy, but push_repo_memory.cjs copies only files present in the artifact and scopes git status/git add to those paths. Missing legacy JSON files therefore are not staged as deletions in the push job, and push-time validation still sees them.

I restored max-file-count: 6, because once deletion propagation is supported a migration can change up to five legacy entries plus notes.json. This PR should remain draft until the required gh-aw deletion-propagation mechanism is implemented or the migration strategy is changed.