chore: evaluate and update workspace MSRV above 1.85

[josecelano]

Summary

Decide on the appropriate Minimum Supported Rust Version (MSRV) policy for the workspace and update rust-version in Cargo.toml accordingly.

Background

PR #1784 set rust-version = "1.85" — the strict minimum required for Rust edition 2024. This is now several releases behind current stable.

Two classes of crate coexist in the workspace:

• Application layer (torrust-tracker-*, main binary) — no downstream MSRV impact
• Protocol/domain layer (bittorrent-* crates) — planned for extraction as independent crates.io libraries, where a conservative MSRV matters for downstream BitTorrent projects

A rational MSRV policy requires knowing: which packages will be extracted, their final names, versioning lifecycle, and publication targets.

Blocked on #1669

This issue is blocked on #1669 (ongoing package restructuring). The decisions needed to set a rational MSRV policy (extraction scope, names, versioning lifecycle) have not yet been made. Re-evaluate after #1669 is settled.

Spec

Full implementation spec: docs/issues/open/ (after this issue is created the draft will be moved there)

Tasks (summary, to be executed after #1669)

• T1: Decide MSRV policy with rationale
• T2: Update rust-version in root Cargo.toml
• T3: Update AGENTS.md MSRV reference
• T4: Update setup-dev-environment SKILL.md MSRV reference
• T5: Verify CI passes

Related to #1784
Blocked by #1669