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/fish.fish

@@ -0,0 +1,32 @@
+function jjf
+    set -e _jjf_pending_swap
+    # Meta-commands and bare flags bypass the picker and run the binary directly.
+    if test (count $argv) -eq 0
+        command jjf
+        return $status
+    end
+    if test "$argv[1]" = init; or string match -q -- '-*' $argv[1]
+        command jjf $argv
+        return $status
+    end
+    set -l _jjf_cmd (command jjf --emit $argv)
+    set -l _jjf_status $status
+    if test $_jjf_status -ne 0
+        return $_jjf_status
+    end
+    if test -z "$_jjf_cmd"
+        return 130
+    end
+    builtin history append -- $_jjf_cmd
+    # Signal the postexec hook to remove the literal `jjf …` entry that fish
+    # is about to record (fish records function invocations *after* return).
+    set -g _jjf_pending_swap 1
+    sh -c $_jjf_cmd
+end
+
+function _jjf_postexec --on-event fish_postexec
+    if test "$_jjf_pending_swap" = "1"
+        builtin history delete --exact --case-sensitive -- $argv[1]
+        set -e _jjf_pending_swap
+    end
+end