Support @file context in custom command arguments

[emecii]

Summary

• resolve @file custom-command and parameterized-skill arguments after substitution and attach their contents through the existing prompt-context format
• share path/range parsing with regular prompt contexts, including ~ expansion and trailing ranges on Windows-style paths
• preserve quoted arguments containing spaces and document the quoting requirement

Fixes #163

Testing

• clojure -M:test --focus eca.features.commands-test — 27 tests, 199 assertions
• clojure -M:test --focus eca.features.context-test — 11 tests, 54 assertions
• clojure -M:test --focus eca.features.chat-test — 72 tests, 468 assertions
• bb test — 934 tests, 5,374 assertions
• bb integration-test --dev --ns integration.chat.commands-test — 4 tests, 74 assertions
• clj-kondo on changed source/tests — 0 errors, 0 warnings
• Whole-tree clj-kondo only reports the pre-existing unresolved dir in test/eca/features/hooks_test.clj:259

This contribution was prepared with OpenAI Codex assistance.

• Added an Unreleased changelog entry.

[zikajk]

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