fix(cli): keep help and schema examples consistent

[jo-hnny]

Summary

• Parse the final Cobra Example blocks back into command examples and use them to populate CommandSchema.Examples.
• Initialize and include Cobra's special help command when building the schema catalog.
• Add regression coverage for multi-line examples and a gate that keeps every public leaf command's help and schema examples aligned.

Related to #46.

Root cause

Command examples could come from generated descriptors, handwritten Cobra commands, Long help text, or the default example fallback. Text help used the final Cobra command tree, while the schema catalog only copied examples from selected descriptor metadata. As a result, examples visible in help were missing from machine-readable schema output.

External impact

• Plain-text --help, agr help, and text-mode agr schema output are unchanged.
• agr schema ... -o json and --help -o json now expose existing help examples for 37 additional commands.
• Leaf-command example coverage increases from 5/42 to 42/42.
• Command ordering, non-example schema fields, exit codes, command execution, and flag parsing are unchanged.
• This populates the existing Examples field in agr.v1; it does not introduce a new schema field. Consumers using exact JSON snapshots will observe the additional values.

Benefits

• Keeps human-readable help and machine-readable schema consistent from the same final Cobra command state.
• Makes more realistic usage examples available to LLMs and other schema consumers.
• Prevents future commands from silently drifting through an all-command regression gate.
• Reuses existing example definitions instead of duplicating them in handwritten schema metadata.

Validation

• go test ./... -count=1
• go run ./cmd/internal/cobragen check
• git diff --check
• Compared pre-change and post-change CLI output: all 52 command help pages and root text help are unchanged; only Examples values differ in JSON schema/help output.