Support @file context in custom command arguments

[emecii]

Summary

• resolve @file custom-command arguments after variable substitution and attach their contents through the existing prompt-context format
• preserve tokenized paths with spaces and line ranges, while leaving the prompt unchanged when a referenced file is unreadable
• document the behavior and add focused regression coverage

Fixes #163

Testing

• clojure -M:test — 928 tests, 5,274 assertions, 0 failures
• clojure -M:test --focus eca.features.commands-test — 26 tests, 196 assertions, 0 failures
• bb integration-test --dev --ns integration.chat.commands-test — 4 tests, 74 assertions, 0 failures
• clj-kondo --lint src/eca/features/commands.clj test/eca/features/commands_test.clj — 0 errors, 0 warnings

Whole-tree clj-kondo still reports the pre-existing unresolved dir symbol in test/eca/features/hooks_test.clj:259; the same result reproduces on clean master.

This contribution was prepared with AI assistance. The implementation was exercised with the tests and lint commands listed above.

• I added a entry in changelog under unreleased section.
• This is not an AI slop.

[zikajk]

@emecii Nice feature, and the happy path is solid! Just one blocker.