perf: skip syntax highlighting for agent

[slarse]

Vibed, not yet looked through this. But we should save plenty of performance inbut diff by not syntax highlighting.

Benchmarks will include AI output mode from tomorrow's run, so this should go in at the very earliest after that.

[Copilot]

🟢 Approval recommended

No unresolved review issues were identified.

Pull request overview

Optimizes but diff for agent output by skipping syntax highlighting while preserving diff colors and layout.

Changes:

• Selects plain rendering for agent mode.
• Adds tab expansion support.
• Adds human and agent snapshot coverage.

File summaries

File	Description
crates/but/tests/but/command/snapshots/diff/human-syntax.stdout.term.svg	Human syntax-highlighted output snapshot.
crates/but/tests/but/command/snapshots/diff/agent-no-syntax.stdout.term.svg	Uncommitted agent output snapshot.
crates/but/tests/but/command/snapshots/diff/agent-committed-no-syntax.stdout.term.svg	Committed agent output snapshot.
crates/but/tests/but/command/diff.rs	Agent rendering regression tests.
crates/but/src/utils/diff_rendering.rs	Tab expansion support for plain rendering.
crates/but/src/command/legacy/diff.rs	Selects highlighted or plain rendering based on agent mode.

Review details

• Files reviewed: 3/6 changed files
• Comments generated: 0
• Review effort level: Lite

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