feat(dispatch): run rtk as a busybox-style multi-call binary via argv[0]

[mazzz1y]

Summary

• Run rtk as a busybox-style multi-call binary: when invoked under a tool's name via a symlink (e.g. head -> rtk), it rewrites the invocation to the equivalent rtk <sub> via the rewrite registry, and execs the real tool when there's no equivalent (or the synthesized form fails to parse)
• Self-excluding binary resolution (resolve_binary skips PATH candidates that are symlinks resolving to a file named rtk) so a tool -> rtk symlink can't re-exec rtk forever — it always finds the genuine tool further down PATH
• The arg tail is spliced from the original OsString args (not re-split from a shell string), so quotes/escapes/non-UTF8 survive verbatim; a lexer round-trip guard passes structured/compound args straight through to the real tool
• Manual usage documented in README.md

Why

I want to wire rtk into a Nix derivation that builds an idempotent, declarative setup across multiple tools on a machine. The auto-rewrite hook needs a Unix shell + per-agent hook config and mutates state at runtime, which doesn't fit a pure/idempotent Nix build.

With argv0 dispatch I can declaratively create tool -> rtk symlinks (ln -s rtk head) in the derivation output and have those tools transparently routed through rtk's filters — no hook, no runtime mutation, reproducible across hosts.

Tools rtk doesn't rewrite (or invocations with no equivalent) pass straight through to the real binary, so shadowing them on PATH is safe.

Test plan

# Build, then create tool-name symlinks pointing at the rtk binary.
RTK="$(pwd)/target/debug/rtk"
mkdir -p /tmp/rtk-shims
ln -sf "$RTK" /tmp/rtk-shims/head
ln -sf "$RTK" /tmp/rtk-shims/git
export PATH="/tmp/rtk-shims:$PATH"   # shadow the real tools

# Invoked under a tool's name -> rewritten to the rtk equivalent:
head file.txt           # runs `rtk read file.txt`
git log                 # runs `rtk git log`

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