feat(repl): bring REPL to feature parity with ZSH shell plugin

[tusharmath]

Summary

Bring the REPL to feature parity with the ZSH shell plugin by adding fzf-based file and command pickers, a syntax highlighter, a starship-style prompt redesign, and a unified forge list file command that both the REPL and the shell plugin now share.

Context

The ZSH shell plugin has long had rich completion (fzf + fd, bat preview, colon-prefix commands, highlighted input) that the built-in REPL lacked. Users switching between shell mode and REPL mode experienced a jarring inconsistency. This PR closes that gap by porting the ZSH experience into the REPL itself, while also sharing the underlying file-listing logic through a new forge list file CLI command so the shell plugin no longer needs a separate fd dependency.

Changes

• fzf file picker in REPL — replaces the nucleo fuzzy-matcher with ForgeWidget (fzf). File completions now open an interactive picker with bat-powered syntax-highlighted previews (falls back to cat), matching the shell plugin's @-mention completion experience.
• fzf command picker in REPL — replaced the inline suggestion list for / and : commands with an fzf popup that shows each command name and description side-by-side, making command discovery much easier.
• Colon-prefix command support — :command now works identically to /command in both completion and execution, matching shell plugin behaviour.
• Syntax highlighter — new highlighter.rs that colours slash/colon commands, shell (!cmd) commands, and file mentions ([path]) in real-time as the user types, giving immediate visual feedback.
• Starship-style prompt redesign — the left prompt now shows the git branch (with branch symbol); the right prompt shows agent symbol, model name (provider prefix stripped), token count (humanised, e.g. 1.5k), cost, and dims all labels when the session is inactive (no tokens consumed yet).
• forge list file CLI command — new forge list --file / forge list files subcommand that walks the workspace with the same rules as fd --type f --type d --hidden --exclude .git and prints one path per line. The shell plugin's completion.zsh now delegates to this command instead of calling fd directly, removing the external dependency.
• Walker parallelism and hidden-file support — Walker::get_blocking migrated to WalkBuilder::build_parallel() for better performance on large workspaces. New hidden flag controls dotfile visibility (false = include hidden, matching fd --hidden). .git directory is always excluded.

Key Implementation Details

Walker::max_all() (used by the REPL picker and forge list file) sets hidden: false, so dotfiles are included just like fd --hidden --exclude .git. Walker::default() keeps hidden: true for agents that should not see dotfiles. The parallel walker uses Arc<Mutex<…>> shared state so all threads contribute to the same collection and global limits. The fzf command picker builds CommandRow display objects (name padded to 30 chars + description) and pre-populates the fzf query with whatever the user has already typed after the sentinel.

Use Cases

# Tab-complete a file mention — opens fzf picker with bat preview
> Tell me about @src/<TAB>

# Tab-complete a command — opens fzf picker showing name + description
> /<TAB>   or   :<TAB>

# Real-time highlighting while typing
> /ask explain this [src/main.rs]   ← command is green, file is cyan

# List workspace files from any script or shell alias
$ forge list file --porcelain | fzf

Testing

# Run all crate tests (snapshot tests auto-accepted)
cargo insta test --accept

# Manual REPL smoke-test
cargo run -- repl
# 1. Type `@` then Tab → fzf file picker should open with preview
# 2. Type `/` then Tab → fzf command picker should open
# 3. Type `:` then Tab → same fzf command picker, colon sentinel preserved
# 4. Observe syntax highlighting as you type a slash command + file mention

# Verify the new CLI command
cargo run -- list file
cargo run -- list file --porcelain

Links

• Related PR (original fzf file picker): feat(repl): bring REPL to feature parity with ZSH shell plugin #2984