feat: add --generate-completion for shell tab completion

- Add clap_complete dependency for bash/zsh/fish/elvish/powershell
- Add --generate-completion <shell> flag that prints completion script to stdout
- profile.bash sources completions via command keep --generate-completion bash
- @ and @@ aliases get completions via wrapper functions that delegate to _keep
- README updated with Shell Completion section

Cargo.lock

@@ -435,6 +435,15 @@ dependencies = [
  "strsim",
 ]
 
+[[package]]
+name = "clap_complete"
+version = "4.6.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "19c9f1dde76b736e3681f28cec9d5a61299cbaae0fce80a68e43724ad56031eb"
+dependencies = [
+ "clap",
+]
+
 [[package]]
 name = "clap_derive"
 version = "4.6.0"
@@ -1649,6 +1658,7 @@ dependencies = [
  "base64",
  "chrono",
  "clap",
+ "clap_complete",
  "comfy-table",
  "config",
  "ctor",