feat: add {PORT}, {HOST}, {PORTLESS_URL} placeholders for command args

[oosawy]

Summary

• Many tools ignore the PORT env var and require CLI flags instead, with no standard flag name across tools
• The existing injectFrameworkFlags whitelist can't cover all tools, and Vite-based tools with their own CLI keep growing
• Users can't use $PORT in command args because the shell expands it before portless assigns a port; sh -c "..." works but is awkward
• {PORT}, {HOST}, and {PORTLESS_URL} placeholders in command args are now replaced with the assigned values before the command runs:

  portless run my-server --port {PORT} --host {HOST}
  

• When placeholders are present, automatic --port/--host injection is skipped
• {...} syntax has no special meaning in bash, zsh, fish, PowerShell, or cmd.exe, so it passes through to portless as-is

Implementation Details

• hasPlaceholders() and replacePlaceholders() added to cli-utils.ts; exact-match only, uppercase only, unknown tokens ignored
• In runApp(), placeholders are replaced after port assignment and before injectFrameworkFlags; if any placeholder was present, framework injection is skipped entirely
• PORT/HOST/PORTLESS_URL env vars are still set regardless of placeholder usage

Test plan

• pnpm exec vitest run src/cli-utils.test.ts -- 92 tests pass (16 new for hasPlaceholders/replacePlaceholders)
• portless run my-server --port {PORT} -- confirm port is replaced and server starts on the assigned port
• portless run vite dev -- confirm auto-injection still works when no placeholders are used
• Verify --help output includes the new Placeholders section

[vercel]

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