build(lint): enforce the AGENTS.md size and complexity rules

[furedea-agent]

Changes

• clippy.toml (new): too-many-arguments-threshold = 3, excessive-nesting-threshold = 2, cognitive-complexity-threshold = 15.
• Cargo.toml [lints.clippy]: warn on allow_attributes_without_reason (restriction), cognitive_complexity (nursery), and excessive_nesting (restriction). too_many_arguments is already on through clippy::all.
• lefthook.yml: new file-length pre-commit command that rejects staged *.rs files over 500 lines.

Tool versions checked

• clippy 0.1.91: all three clippy.toml keys are accepted (clippy rejects unknown keys) and all lint names exist. A probe crate with four arguments, three nested ifs, and a reason-less #[allow] triggered too_many_arguments (4/3), excessive_nesting, and allow_attributes_without_reason.
• lefthook 2.1.15.

Suppressions

None. The template code already passes the new rules.

Checks run

• lefthook run pre-commit --all-files (ls-lint, file-length, deadnix, statix, nixfmt, fmt, clippy): pass
• cargo clippy -- -D warnings: pass
• cargo fmt --check: pass
• cargo test: pass
• cargo machete: pass
• cargo deny check bans licenses sources: pass (the advisories check could not run locally because the sandbox makes ~/.cargo/advisory-dbs read-only; CI covers it)

Refs furedea/dotfiles#213

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