fix(command-parser): user-friendly errors for unknown commands and consistent list command output

[tusharmath]

Summary

Improve the command-line and REPL experience by showing user-friendly error messages for unknown commands, fixing list command output, and adding a help action to the shell plugin.

Context

Previously, when a user typed an unknown command like :celar, the error message surfaced raw Clap internals (e.g. references to forge_cmd) with no guidance on how to recover. The list command output was also inconsistent: in non-porcelain mode it built a full Info table including agents and custom commands, while :help in the REPL used a different code path — so the two outputs diverged. The shell plugin also had no way to show available commands outside the REPL.

Changes

• User-friendly unknown-command errors: When a user types an unrecognised command (e.g. :celar or /celar), the error now reads "Unknown command ':celar'. Run ':help' to list available commands." instead of a raw Clap error.
• Cleaned-up Clap error output: The internal binary name forge_cmd is replaced with forge in all other Clap error messages.
• Consistent list command output: Non-porcelain mode now delegates to ForgeCommandManager (the same path as :help), so forge list command and :help always render identically.
• Shell plugin help action: Added _forge_action_help and wired it to the help keyword in the dispatcher so users can run help in the shell-plugin context and get the same output as :help.
• Posthog dependency bump: Updated the posthog-rs git revision.

Key Implementation Details

• The command prefix (/ or :) is captured before stripping it from the token, so the error message reflects exactly what the user typed.
• Porcelain mode still builds the full Info table (built-ins + agent aliases + agent infos + custom commands) for shell-completion consumers; non-porcelain mode uses ForgeCommandManager::default() directly.
• Two new unit tests cover the helpful error for both : and / prefixes.

Testing

# Verify unknown-command error messages
cargo insta test --accept -p forge_main

# Try it interactively
cargo run -- chat
# Then type:  :celar
# Expected:   Unknown command ':celar'. Run ':help' to list available commands.

# Verify list command (non-porcelain)
cargo run -- list command

# Verify list command (porcelain)
cargo run -- list command --porcelain