ATUI-10: Quoted paths, literal @ escape, multi-ref failure aggregation

[guimorg]

What to build

Add ergonomics on top of the base @file reference mechanism:

• Quoted paths: @"path with spaces.md" accepts paths containing whitespace or shell-special characters. Unmatched quotes error clearly.
• Literal escape: \@foo sends a literal @foo in the message rather than resolving it. user@example.com, @ followed by whitespace, and @ at end of prompt remain non-special without an escape.
• Multi-ref failure aggregation: when a prompt contains multiple @refs and one or more fail to resolve, the error output lists all failures at once. The prompt is not sent to ask_adapt. The REPL input line is preserved so the user can edit rather than retype.

Acceptance criteria

• @"my notes.md" in a prompt resolves the space-containing path correctly
• An unmatched @"... produces a clear parse error and prevents sending
• \@foo sends a literal @foo to ask_adapt; the parser does not attempt to resolve it
• user@example.com in a prompt is unaffected (no attempted resolution)
• A trailing @ with no following token is unaffected
• A prompt with three failing @refs produces one error listing all three, not three separate turns
• After a failure, the REPL input line preserves the user's typed content for editing (not blanked)
• Tests cover: quoted path with spaces, unmatched quote, \@ escape, mid-word @, trailing @, and multi-failure aggregation with 2+ refs

Blocked by

• ATUI-9: Deny-list, absolute paths, tilde, symlink canonicalization #14 (ATUI-9: Deny-list, absolute paths, tilde, symlink canonicalization)