Clarify Assisted-by trailer format in AGENTS.md

[L0Lmaker]

Summary

• The current template Assisted-by: AGENT_NAME:MODEL_VERSION [TOOL1] [TOOL2] is being interpreted inconsistently — at least two contributors have placed the model family in the agent slot and the IDE/CLI in brackets, when the intended structure (per @spmallette) is <agent>:<model> with brackets reserved for auxiliary tools.
• Names each placeholder explicitly and adds three concrete examples in the prose covering the common single-agent case and the rarer auxiliary-tool case.
• Style matches the rest of AGENTS.md — inline backticks throughout, no fenced or indented code blocks introduced.

Context

Raised by @spmallette on #3416:

I'd envisioned: Assisted-by: Claude Code:claude-opus-4-7 ... I might need to better define that to get what I want there. It's interpreting the "tool" to be Claude Code - maybe because the tool is not seen as optional??

This PR addresses that by naming each placeholder and providing examples that demonstrate the intended form. The bracket interpretation in the new wording — "auxiliary tools beyond the primary agent" — reflects my reading; happy to revise if a different meaning is preferred.

Branches

AGENTS.md exists only on master, so this is master-only.

Test plan

• Renders correctly on GitHub
• No fenced or indented code blocks introduced — existing AGENTS.md prose-with-inline-backticks style preserved
• All in-prose examples conform to the new format

Notes

This change was AI-assisted; the commit's Assisted-by: trailer demonstrates the new format.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 76.25%. Comparing base (cfd6889) to head (cbffc7a).
⚠️ Report is 1033 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #3417      +/-   ##
============================================
- Coverage     77.87%   76.25%   -1.62%     
+ Complexity    13578    13319     -259     
============================================
  Files          1015     1011       -4     
  Lines         59308    59941     +633     
  Branches       6835     7011     +176     
============================================
- Hits          46184    45708     -476     
- Misses        10817    11540     +723     
- Partials       2307     2693     +386     

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[spmallette]

thanks - hopefully that will help keep that line looking how we want.