feat(config): auto-migrate deprecated [llm] to [agent.llm]

[Shearerbeard]

Summary

• Old configs with top-level [llm] now auto-migrate to [agent.llm] at load time instead of failing
• Deprecation warning logged so users know to update
• Ambiguous configs (both [llm] and [agent.llm]) and missing [agent] still fail with clear errors
• Quickstart example config updated to new shape

[promptless]

Promptless prepared a documentation update related to this change.

Triggered by mezmo/aura#142

Updated docs/breaking-changes/20260421-llm-under-agent.md to reflect the auto-migration behavior: changed the summary to indicate configs are auto-migrated with a deprecation warning, added the exact warning message users will see, and documented the two cases that still fail (ambiguous configuration and missing [agent] section).

Review: Update LLM migration docs for auto-migration behavior

[andrewelizondo]

A few small suggestions from a review pass:

1. Stale doc comment on migrate_legacy_top_level_llm: the doc still frames the function as emitting a migration error ("Detect the legacy top-level [llm] table shape and emit a migration error") and the second paragraph explains the silent-ignore rationale that predates this PR. Worth rewriting to reflect that it now auto-migrates and only errors on ambiguous / missing-[agent] cases.

2. Missing test for [llm.additional_params] migration: the previous hard-error message specifically called out [llm.additional_params] as a thing users have, and it's a common shape (temperature, max_tokens, etc.). Since the whole toml::Value is moved it should migrate correctly, but there's no test asserting it does. A regression here would be easy to miss — the renamed test_legacy_top_level_llm_migrated_with_warning only checks api_key and model.

3. Pin a removal version in the deprecation warning: the current message says "this fallback will be removed in a future release" without a target. Naming a version (e.g. "removed in v1.22") makes the deprecation actionable for users reading the warning in logs.