feat: add at_filename usage for users to directly involve a filepath to context

[Dong1017]

Summary

Add @filename prompt support so users can reference workspace files directly from chat and selected slash commands.

What’s Included

• Added @relative/path expansion for:
  • Plain chat
  • /report
  • /diagnose
  • /fix
  • /skill <name> ...
  • /<skill> ... aliases
• Added @file suggestions in the input composer
• Added backend validation for referenced files
• Expanded @file into a workspace file marker:
  • [file path="<absolute workspace path>"]
• Let the agent decide whether to read file contents through filesystem tools instead of auto-inlining content

Behavior

• Only standalone whitespace-delimited @relative/path tokens expand
• @@name keeps a literal @name
• Paths must stay inside the workspace
• Referenced targets must exist and be files
• Invalid @file references fail the whole input

Implementation

• Added input expansion flow for supported command surfaces
• Added shared workspace file path validation
• Updated tools/fs path handling so agent-side reads work with workspace absolute paths
• Updated help text and docs for the new @filename workflow

Files Changed

• internal/app/input_expansion.go
• internal/workspacefile/workspacefile.go
• tools/fs/pathutil.go
• tools/fs/pathutil_test.go
• internal/app/commands.go
• internal/app/input_expansion_test.go
• ui/app_train_test.go
• docs/at-file-input-expansion-summary.md

Testing

• go test ./internal/app -run 'Test(ExpandInputText|ProcessInput|HandleCommand)'
• go test ./tools/fs -run 'TestResolveSafePath'
• go test ./ui -run 'TestAtFileInputWaitsForBackendEchoBeforeShowingUserMessage|TestEnterAcceptsAtFileSuggestionBeforeSubmittingToBackend'