Bug: .github/agents imports on Claude/Codex take fragile prompt-assembly path; .github/snippets avoids it

## Summary
Docs say `.github/agents/*.md` imports should work with non-Copilot engines by injecting markdown body into the prompt, but in practice this path still uses special prompt assembly (`AGENT_CONTENT` + `PROMPT_TEXT`) and remote-merge behavior that appears brittle.

In our CI this manifested as:

`Error: Input must be provided either through stdin or as a prompt argument when using --print`

Switching the same content from `../agents/code-review.md` to `../snippets/code-review.md` removed the special path and resolved the failure.

## Environment
- `gh aw version v0.53.1` (2026-03-04)

## Docs vs Behavior
### Docs indicate `.github/agents` should work for Claude/Codex by injecting markdown body
- https://github.com/github/gh-aw/blob/main/docs/src/content/docs/reference/copilot-custom-agents.md#L8
- https://github.com/github/gh-aw/blob/main/docs/src/content/docs/reference/imports.md#L76-L120

### Current compiler still emits special agent assembly path
- Claude: `AGENT_CONTENT` + `PROMPT_TEXT`
- https://github.com/github/gh-aw/blob/main/pkg/workflow/claude_engine.go#L221-L234
- Codex: similar path
- https://github.com/github/gh-aw/blob/main/pkg/workflow/codex_engine.go#L215-L222
- Main job still emits `merge_remote_agent_github_folder` for agent imports
- https://github.com/github/gh-aw/blob/main/pkg/workflow/compiler_yaml_main_job.go#L102-L113

## Minimal Repro
Create two workflows with identical content except import path:

1. `imports: ../agents/code-review.md`
2. `imports: ../snippets/code-review.md`

Compile both with `gh aw compile`.

### Observed
`../agents/...` lockfile includes:
- `AGENT_CONTENT=...awk...`
- `PROMPT_TEXT=...printf...`
- `merge_remote_agent_github_folder`

`../snippets/...` lockfile does **not** include those pieces.

## Why this seems buggy
If docs guarantee `.github/agents` as valid prompt injection for Claude/Codex, behavior should be as robust as snippet imports. Today, `.github/agents` appears to trigger extra legacy/special handling with different failure modes.

## Related real-world diff
This PR shows the practical workaround that fixed CI by moving import path from `agents` to `snippets`:
- https://github.com/pulumi/pulumi-awsx/pull/1889/changes

## Request
- Confirm whether this is expected behavior or a bug.
- If bug: align `.github/agents` handling with robust snippet-style prompt path for Claude/Codex (or document explicit limitations/workarounds).