compile: directory argument silently no-ops instead of auto-discovering or erroring

## Summary

`ado-aw compile ` silently produces `0 compiled, N skipped` instead of either (a) auto-discovering the `*.md` files in that directory, or (b) failing loudly with a clear "expected a file, got a directory" error.

## Repro

```bash
$ ado-aw compile tests/safe-outputs/
# Detects 26 lock files, reports "(up to date)" for each, then
# emits "Warning: source ... not found for ... skipping" for each,
# and exits 0 with "0 compiled, 26 skipped, 0 failed".
```

`tests/safe-outputs/README.md` (line 102–107) currently documents this exact form as the canonical recompile command:

```bash
# Recompile every fixture in this directory (idempotent):
cargo run -- compile tests/safe-outputs/
```

…so the README is also out of sync with actual behavior.

## What works today

- `ado-aw compile ` — single file, works as documented.
- `ado-aw compile` (no args) — autodiscovers from cwd, works as documented.
- Per-file loop: `for md in tests/safe-outputs/*.md; do ado-aw compile --force "$md"; done` — works but is awkward.
- `cd tests/safe-outputs/ && ado-aw compile` — autodiscovers the 26 lock files but their `source: tests/safe-outputs/...` headers are relative to repo root, so each one is skipped with a "source not found" warning.

## Observed in the wild

The `recompile-safe-output-fixtures` agentic workflow (PR #863, fixed in #864) was triggered against release `v0.31.1` and silently did nothing because it used the documented `compile tests/safe-outputs/` form. Full log: https://github.com/githubnext/ado-aw/actions/runs/27020309715/job/79746499042.

The agent then correctly inspected the lock files, saw `version=0.31.1` matching the latest release, and emitted `noop` — masking the fact that the compile invocation never did any work. A subsequent integrity check would have caught the masking, but the root cause is the silent skip on a directory argument.

## Proposed behaviour

When `path` resolves to an existing directory:

1. Walk it (non-recursively, or recursively — match the autodiscovery behaviour) for `*.md` files that are agentic-pipeline sources.
2. Compile each.
3. Print the same per-file summary lines that the no-args form prints.

If walking yields zero candidates, exit with a non-zero code and a clear error rather than `0 compiled, 0 skipped`.

## Less-good alternatives

- **Reject directory args** with a hard error and a hint pointing at the no-args form. Better than the current silent-skip behaviour but worse than DWIM.
- **Update only the README** to document the per-file loop. Doesn't fix the surprise for everyone else.

## Workaround for now

```bash
for md in tests/safe-outputs/*.md; do
ado-aw compile --force "$md"
done
```

This is what the `recompile-safe-output-fixtures` workflow now does (PR forthcoming after this issue is filed).