docs: clarify hook permissions and input cap

[TurboTheTurtle]

Refs #834.

Carries forward the remaining documentation piece from the closed PR after the code changes were superseded or merged into current master.

Summary:

• show embedding.maxInputChars in the full config example
• clarify hooks.allowConversationAccess for custom/non-bundled plugin entries that override manifest hook metadata

[AliceLJY]

Verified against source: embedding.maxInputChars is a real config field (embedder.ts + manifest schema; default derives from the model, so 1400 works as an explicit example), and the hooks.allowConversationAccess guidance matches the hook-gating behavior discussed in #848/#850. Useful doc coverage for the non-bundled install case. LGTM.

[rwmjhb]

Approved on head 85adeb9.

The orchestrator value gate passed for this docs-only change; the later R2a model call hung before producing findings, so I completed the review manually against the README diff.

Independent verification run on the same head:

• npm ci --silent
• npm run build --if-present
• git diff --check
• npm test
• git diff --exit-code -- dist src index.ts package.json package-lock.json openclaw.plugin.json README.md scripts test

All passed. The README additions match existing config behavior: embedding.maxInputChars is already supported, and the hook permission guidance is scoped to custom/non-bundled entries overriding manifest hook metadata.