docs: never mention the grug handle on GitHub

[quadseven]

Refs #68.

Why

Two "re-review" requests on this repository (#62 and #69) were posted as an
at-mention of the name grug. That handle belongs to a real GitHub user
unrelated to this project, so both notified them and subscribed them to the
threads, which cannot be undone. The review bot is grug-tribe[bot] and is
driven by slash commands, not mentions. Cloud sessions that see only this
repository need the rule in the repository itself.

What changed

A short paragraph near the top of AGENTS.md: never at-mention that handle
anywhere on GitHub; the bot's commands are /grug improve (re-run code
review) and /grug recheck (re-run the plan check); write grug in backticks
in prose; check text before posting.

Acceptance criteria

• The rule sits near the top of AGENTS.md, where every reader starts
• It names the correct slash commands and does not name the person
• This PR's title, body and commit message contain no at-mention of the handle (checked with grep before posting)

Size: XS

Out of scope

The same rule in other repositories' agent files.

https://claude.ai/code/session_01UkWktkaYXZwdwzUvZQrN9q

[grug-tribe]

Walkthrough

The PR adds a warning to AGENTS.md instructing contributors never to write a bare @​grug mention on GitHub, because it refers to an unrelated real user who gets notified and auto-subscribed (happened twice in issues #62 and #69). It clarifies that the review bot is grug-tribe[bot] and responds to slash commands like /grug improve and /grug recheck, and recommends using backticks around grug in prose and double-checking any text for stray @​grug before posting.

Review effort: quick (~5 min)

Changed files (1)

File	Summary	Changes
AGENTS.md	Added guidance warning against using the bare @​grug GitHub handle and documenting the grug-tribe[bot] slash commands.	+8/-0

Shape of the change

graph TD
  subgraph "root"
    N0["AGENTS.md"]
  end

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