fix: migrate recall to before_prompt_build

[vincentkoc]

Summary

• register OpenClaw recall only through before_prompt_build
• keep the legacy before_agent_start hook only for the separately advertised Moltbot and ClawDBot entrypoints
• declare the minimum supported OpenClaw plugin API and align docs, config UI copy, tests, and release-note generation

Why

OpenClaw retired before_agent_start. The version-path fallback could still register that removed hook on current development builds whose entrypoint did not expose a parseable package version, causing plugin inspection and runtime compatibility failures.

Verification

• GIT_CONFIG_COUNT=1 GIT_CONFIG_KEY_0=tag.gpgSign GIT_CONFIG_VALUE_0=false npm test
• npm pack --dry-run
• git diff --check

Found through the OpenClaw HEAD compatibility canary in openclaw/crabpot.

[wacke1]

This PR (migrate recall to before_prompt_build) is related to the DuplicateOpenClawRuntimeError issue (#159). Gateway has been stabilized with MemoryHigh=5G, MemoryMax=6G. No restarts in 24h+. Please verify the PR does not reintroduce the runtime-lock issue after merging.