Fix MCP transparent fallback mode

[wbugitlab1]

Summary

• Add storage_mode: "local" | "remote" to successful JSON text responses for the seven fallback-capable standalone MCP shim tools.
• Add AGENTMEMORY_NO_FALLBACK=1 as a strict/no-local-fallback alias alongside the existing strict flags.
• Update README/package/config docs and regenerated config-skill reference.

This addresses the transparent fallback and strict alias parts of #273. Sync-on-reconnect remains intentionally out of scope.

Verification

• corepack pnpm run skills:gen
• corepack pnpm run skills:check
• corepack pnpm run lint
• corepack pnpm run build
• corepack pnpm test
• git diff --check
• semgrep scan --config p/default --error --metrics=off .
• gitleaks protect --staged --redact
• gitleaks detect --source . --redact --log-opts origin/main..HEAD

Notes:

• Full test suite passed after the latest origin/main merge: 207 files, 2833 tests.
• PR-range Gitleaks passed. A separate full-history scan surfaced historical baseline findings outside this PR commit range.