feat(cli): add examples to ao preview --help

[i-trytoohard]

What

Improves ao preview --help with concrete examples for the common workflows.

Closes #387

Changes

• Added Example field to the preview command — renders an Examples: section in --help with the four common workflows:

  ao preview
  ao preview file://$(pwd)/index.html
  ao preview http://localhost:5173
  ao preview clear

• Updated Long description — replaced the workspace-relative path (./dist/index.html) with the absolute file:// URL pattern agents actually use.
• Added test (TestPreview_HelpIncludesExamples) verifying the examples section is present, the file:// example exists, and the old relative-path reference is gone.

Acceptance criteria

Criterion	Status
ao preview --help includes a concise examples section	✅
File example uses file://$(pwd)/index.html, not ./index.html	✅
Existing behavior and command syntax unchanged	✅

Test output

=== RUN   TestPreview_WithURLArg              --- PASS
=== RUN   TestPreview_NoArgPostsEmptyURL      --- PASS
=== RUN   TestPreviewClear_DeletesSessionPreview --- PASS
=== RUN   TestPreviewClear_MissingSessionIDIsUsageError --- PASS
=== RUN   TestPreview_MissingSessionIDIsUsageError --- PASS
=== RUN   TestPreview_HelpIncludesExamples    --- PASS
=== RUN   TestPreview_BlankSessionIDIsUsageError --- PASS
PASS

[harshitsinghbhandari]

PR Review: #388

Consumer Impact

• backend/internal/cli/preview.go:26 says bare ao preview “opens the workspace’s index.html.” That is incomplete for the real daemon behavior: setPreview first autodetects a workspace entry, but if none exists it falls back to the session’s existing preview target. There is already a controller test for that fallback: TestSessionsAPI_SetPreviewEmptyURLReusesExistingTargetWhenNoEntryExists. The help text should preserve that nuance while still using the better file://$(pwd)/index.html example.

Suggested wording:

"With no argument it opens the workspace's static entry point, falling back\n" +
"to this session's existing preview target when no entry point exists. A local\n" +
"file can be opened by its absolute file:// URL (e.g. file:///home/me/proj/index.html).\n" +

Boundary Verification

Skipped. The production change is help text only: no new function calls, imports, file I/O, network calls, serialization, schemas, or behavior changes. The boundary risk is the human-facing CLI help contract, covered above.

Testing

I ran:

go test ./internal/cli -run 'TestPreview' -count=1
go test ./internal/cli -count=1
go run ./cmd/ao preview --help
git diff --check origin/main...HEAD

All passed. The rendered help includes the requested ao preview file://$(pwd)/index.html example.

Recommendation

Request Changes

The requested example is good, but the no-argument behavior description regressed from slightly over-specific to still inaccurate in a different way. Fixing that wording should be enough.