When cwd was passed with ~ or $HOME (e.g. ~/src/myproj), it was used
unexpanded in BuildAgentEnv -> resolvePrompt, causing prompt commands
to fail silently (invalid cmd.Dir) and producing an empty system prompt.
Apply os.ExpandEnv + paths.ExpandHome before cwd is used anywhere.
Replaces polling with a blocking file that returns new bytes as they
are appended to the session log. Unblocks immediately on end-of-turn
(EnsureTrailingNewline) so frontends exit cleanly.
Levi Neely
Logan Neely
Levi Neely