Add tab completion for commands and file paths

Implement tab completion in interactive mode using rustyline:

- Command completion: /<TAB> shows all commands, /com<TAB> -> /compact
- File path completion: /run <TAB> completes file/directory paths
- Supports tilde expansion for home directory

Architecture is extensible for future semantic completions:
- /resume <TAB> -> session IDs (Phase 2)
- /rehydrate <TAB> -> fragment IDs (Phase 2)

New module: completion.rs with G3Helper struct implementing
rustyline's Completer trait.

crates/g3-cli/Cargo.toml

@@ -17,7 +17,7 @@ tracing = { workspace = true }
 tracing-subscriber = { workspace = true, features = ["env-filter"] }
 serde = { workspace = true, features = ["derive"] }
 serde_json = { workspace = true }
-rustyline = "17.0.1"
+rustyline = { version = "17.0.1", features = ["derive", "with-dirs"] }
 dirs = "5.0"
 tokio-util = "0.7"
 sha2 = "0.10"