feat(cli): add bun and bunx command support

[danielctc]

What

Adds rtk bun and rtk bunx support, mirroring the existing npm/npx/pnpm handlers.

Closes #832. Also addresses #1316, #599, #1735, #1305, #930, #1802, #197.

Why

bun is one of the most common unhandled commands surfaced by rtk discover — on a bun-first project bun run alone shows ~350 invocations/month — yet npm, npx and pnpm are all routed. This closes the gap for the Bun ecosystem.

How

• Rewrite rules (src/discover/rules.rs) — bun (run|x|test) → rtk bun, bunx … → rtk bunx, both PackageManager / 70% like the npm/npx rules.
• Filter (src/cmds/js/bun_cmd.rs) — strips bun's bun run v… / bun test v… banner and the $ <cmd> echo line (bun's equivalent of npm's > pkg@ver boilerplate); collapses blank lines; returns ok when nothing remains. Auto-discovered via automod.
• Dispatch (src/main.rs) — Bun/Bunx clap subcommands. Bunx intelligently routes tsc/eslint/prisma/next/prettier/playwright into the existing specialised filters exactly as Npx does; everything else falls through the bunx filter.
• Telemetry (src/core/tracking.rs) — bun/bunx categorised under js so rtk gain attributes their savings.
• Docs — README Package Managers section.

Design notes

• bun build and bun install are deliberately NOT rewritten — bun build is bun's bundler and bun install is the installer; neither is a script run, and filtering their output would be surprising. The bun (run|x|test) pattern scopes the rewrite to script execution only. Tests assert this.
• No run injection (unlike npm). Because the rewrite only routes bun run|x|test, the bun subcommand is already present in the args — and injecting run would wrongly turn bun build into a script run.
• Smart-routing is done at the bunx dispatch layer (mirroring npx) rather than by expanding the per-tool rewrite regexes, to keep the diff focused. Extending those patterns so rtk rewrite "bunx tsc" resolves directly to rtk tsc can follow if preferred.

Tests

8 new tests, all passing:

• test_rewrite_bun / test_rewrite_bunx — rewrite mapping, plus negative assertions that bun build / bun install are left untouched.
• test_filter_bun_output / test_filter_bun_test_banner / test_filter_bun_output_empty — filter behaviour.

cargo fmt --check and cargo clippy clean.

Note: 3 pre-existing curl_cmd test failures on the base branch ("bytes total" assertions) are unrelated to this change — curl_cmd.rs is untouched here.

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