Initialize fresh Repo Assist memory

[dsyme]

Summary

• initialize a schema-valid notes.json when the fresh Repo Assist memory branch is empty
• leave existing memory and all legacy branches untouched
• document first-run initialization

Failure addressed

The first runs using memory/repo-assist-memory failed because the new orphan branch contained only .git, while the custom validator requires notes.json.

Validation

• gh aw compile workflows/repo-assist.md --strict --validate --no-emit
• first-run initialization fixture parsed with jq
• git diff --check

Security review

The compiler reported the existing repo-memory validation script as an addition to the tracked safe-update manifest. The validator is unchanged. This PR adds only a local pre-agent initialization step that writes a fixed JSON document when notes.json is absent; it introduces no secret, action, permission, redirect, or network destination.