feat: initial implementation of jjf

A TUI revision picker for jujutsu (jj). Wraps any jj subcommand with an
inline fuzzy-finder over jj log, then forwards the picked revision(s)
as -r '<id>' to the underlying jj invocation.

Features:
- Inline viewport (no full-screen alt-screen takeover)
- Smart prefix matching: typing the bold portion of a change ID jumps
  to that revision instantly
- Multi-select via Tab; emitted as -r 'A|B|C' revset union
- Preflight detection of subcommands that don't accept -r
- Optional shell integration (jjf init <bash|zsh|fish|nu>) that
  rewrites shell history so up-arrow recalls the resolved jj command

Hand-rolled ANSI rendering to /dev/tty; only crossterm, nucleo-matcher,
anyhow, and libc as runtime dependencies.

src/init/nu.nu

@@ -0,0 +1,18 @@
+def --env jjf [...args] {
+    # Meta-commands and bare flags bypass the picker and run the binary directly.
+    if ($args | is-empty) {
+        ^jjf
+        return
+    }
+    let first = $args.0
+    if $first == "init" or ($first | str starts-with "-") {
+        ^jjf ...$args
+        return
+    }
+    let result = (^jjf --emit ...$args | complete)
+    if $result.exit_code != 0 { exit $result.exit_code }
+    let cmd = ($result.stdout | str trim)
+    if ($cmd | is-empty) { exit 130 }
+    [{command: $cmd, start_timestamp: (date now | format date "%+")}] | history import
+    ^sh -c $cmd
+}