feat(cli): @path tab-completion in REPL input

[emal-avala]

Summary

@path expansion on submit has worked for a while — typing @src/foo.rs in a prompt inlines the file's contents. But users had to type the full path. This PR adds tab-completion for @ paths anywhere in the input line.

> explain @src/m<TAB>
> explain @src/main.rs                 ← after tab

Rules

• The @ must be at the start of the line or preceded by whitespace, so user@example.com doesn't accidentally trigger path completion mid-word
• Directories are listed first with a trailing / so Tab-tab descends into them
• Dotfiles are hidden unless the partial starts with . (matches shell convention)
• Absolute paths and .. components are rejected — stays within cwd
• Up to 50 candidates returned; ordering: dirs alphabetical, then files alphabetical
• Multiple @ tokens on a line — only the one the cursor is in completes

Display format:

• Files: src/main.rs
• Directories: src/ (dir)

Interaction with existing completers

Slash-command completion (the fuzzy scorer from #195) is unchanged. @ completion only fires when the cursor is in an @ context — slash completion handles everything else. The @ expansion on submit path (from #…) is also unchanged; this PR just makes it easier to produce a valid @path in the first place.

Test plan

• cargo fmt --all — clean
• cargo clippy --workspace --all-targets -- -D warnings — clean
• cargo test -p agent-code --bin agent at_completion_tests — 13/13 pass
  • Context detection: start of line, after whitespace, rejected mid-word, multi-@, whitespace in partial, no @, empty partial (7 tests)
  • Path completion: listing, prefix filter, subdir descent, dotfile gating, traversal rejection, missing dir (6 tests)
• Manual: explain @s<TAB> lists src/, sdks/, etc.; explain @src/m<TAB> expands to @src/main.rs

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