MEM-62: workshop MemWal friction fixes

[ducnmm]

Summary

• Updates SKILL.md and SDK docs for response shapes, *AndWait guidance, namespace/key handling, recall distance filtering, and 401 troubleshooting.
• Adds client-side recall distance filtering support in TS/Python SDKs.
• Renames runtime and example env guidance to MEMWAL_PRIVATE_KEY and removes the old MEMWAL_KEY fallback to avoid credential ambiguity.
• Adds credential verification helpers.

Deployment note

Railway/services that use MemWal memory must set MEMWAL_PRIVATE_KEY now; MEMWAL_KEY is no longer read.

Verification

• ./node_modules/.bin/tsc --noEmit
• ./node_modules/.bin/tsc
• python3 -m pytest packages/python-sdk-memwal/tests/test_client.py -q
• git diff --check

Note: npm latest for @mysten-incubation/memwal is still 0.0.4; local package is already 0.0.5, so no new TS changeset was added to avoid bumping to 0.0.6.