Merge clients into a unified tracker_client CLI (mechanical port)

[josecelano]

Summary

Merge the three existing tracker testing binaries into a single tracker_client binary, while keeping behavior changes minimal:

• http_tracker_client
• udp_tracker_client
• tracker_checker

This is a mechanical unification/refactor, not a CLI redesign.

Context

This work closes the pending EPIC task in #669 (unified client CLI) and follows discussion #660.

• Parent EPIC: Overhaul clients #669
• Discussion: Should we merge testing binaries into a single `Tracker Client`? #660

Decisions for this issue

• CLI shape: explicit protocol subcommands (tracker_client http ..., tracker_client udp ..., tracker_client check ...)
• Output default: JSON (--format=json), with --format=text available
• Legacy binaries: keep for ~1 year with deprecation warning on startup; no new features
• Scope guard: top-level auto-dispatch aliases (tracker_client announce ...) are out of scope for this issue

In Scope

• Implement unified tracker_client entry point with http, udp, check subcommands
• Wire current HTTP/UDP announce/scrape and checker workflows
• Add unified --format=<json|text> behavior
• Add deprecation warnings to legacy binaries
• Update in-repo docs/skills/CI references
• Run and record manual verification scenarios against a local tracker

Out of Scope

• Top-level announce/scrape auto-dispatch aliases by URL scheme (future follow-up)
• REST API console client integration
• Unrelated announce parameter enhancements (HTTP Tracker client: Add optional parameters with the rest of the announce params #1532, UDP Tracker client: Add optional parameters with the rest of the announce params #1533)

Manual Verification (required)

Run local tracker and then execute one manual scenario per command family, tracking status/evidence:

1. HTTP announce
2. HTTP scrape
3. UDP announce
4. UDP scrape
5. Checker
6. HTTP text-format output
7. UDP text-format output

A full matrix with concrete commands and evidence fields is captured in the issue spec.

Follow-up

Create a separate issue for optional top-level announce/scrape convenience aliases that dispatch to HTTP/UDP by URL scheme.