Add Arven Memory provider plugin

[efe-arv]

What Problem This Solves

OpenClaw can select memory backends through bundled plugin metadata and plugins.slots.memory, but there is no bundled Arven Memory option. Users who want to run an Arven-compatible local MCP memory bridge currently need an out-of-tree integration or a UI/config workaround.

This PR adds a bundled arven-memory memory-slot plugin that exposes Arven Memory through an HTTP MCP bridge without patching OpenClaw UI code.

Summary

• Adds extensions/arven-memory with kind: "memory", config schema, UI hints, and tool contracts.
• Registers namespaced Arven tools plus compatibility aliases for memory_search and memory_get when the Arven slot is selected.
• Documents update-agnostic OpenClaw config and legacy provider-name mapping (arven / arven-memory -> plugins.slots.memory = "arven-memory").

Update-agnostic design

This does not edit dropdown/UI code. OpenClaw discovers the provider through normal plugin manifest metadata and exclusive memory-slot selection, so UI/provider list changes should keep picking it up via the existing registry path.

The MCP tool names are configurable (recallTool, getTool, storeTool, statusTool) so bridge implementations can evolve without another OpenClaw UI patch.

Sensitive-data check

Checked the PR diff for common secret patterns, high-entropy strings, tokens, private keys, personal identifiers, and internal references. The only matches are non-secret code/docs references to an optional Authorization header environment variable and loopback example URLs.

Evidence

• CI=true corepack pnpm install --frozen-lockfile
• PATH=/tmp/openclaw-pnpm-shim:$PATH pnpm tsgo
• PATH=/tmp/openclaw-pnpm-shim:$PATH pnpm exec oxlint extensions/arven-memory/index.ts extensions/arven-memory/index.test.ts
• corepack pnpm exec oxfmt --check extensions/arven-memory/index.ts extensions/arven-memory/index.test.ts extensions/arven-memory/package.json extensions/arven-memory/openclaw.plugin.json docs/concepts/memory-arven.md
• PATH=/tmp/openclaw-pnpm-shim:$PATH pnpm vitest run src/plugins/contracts/plugin-tool-contracts.test.ts — 1 file passed, 2 tests passed.
• pre-rebase focused tests passed: corepack pnpm vitest run extensions/arven-memory/index.test.ts
• pre-rebase extension harness passed: PATH=/tmp/openclaw-pnpm-shim:$PATH node scripts/test-extension.mjs arven-memory

Note: after rebasing onto current upstream main, focused extension Vitest spent minutes in upstream plugin timing/build metadata and was stopped before test execution. Full tsgo, lint, format, frozen install, and the tool-contract Vitest passed on the rebased branch.