docs(engines): model field example/comment mismatch

[samuelkahessay]

The "Extended Coding Agent Configuration" example in docs/src/content/docs/reference/engines.md sets an explicit model value but the inline comment describes the default:

model: gpt-5                          # defaults to claude-sonnet-4

The other fields in the same block (e.g. version: latest # defaults to latest) have value and comment in agreement. For model, they describe different things — it's unclear whether the example is demonstrating an override or documenting default behavior.

The workflow schema (pkg/parser/schemas/main_workflow_schema.json) only says engine.model "has sensible defaults and can typically be omitted," so the comment's claim about claude-sonnet-4 is unverifiable from the schema alone.

Suggested fix — clarify the comment as an override example:

model: gpt-5                          # example override; omit to use engine default

Or drop the value to show default behavior:

# model omitted — uses engine default

Verified at v0.62.5 (48d8fdf).