Refactor managed-agent runtime into cohesive modules

[klopez4212]

Summary

• split the managed-agent runtime warehouse into cohesive modules for process ownership/termination, orphan sweeping, dead-instance reaping, lifecycle synchronization, and runtime metadata
• preserve the existing runtime API through narrow re-exports; helper bodies and platform cfg branches are unchanged apart from module-qualified visibility
• reduce runtime.rs from 2,220 lines on main to 908 lines and remove its temporary file-size override, restoring the standard 1,000-line ceiling

Why

main failed after stale successful PR checks allowed independent growth to combine above runtime.rs's 2,216-line override. The earlier fix in #2974 extracted only 55 lines and left the monolith on a special ratchet. This replacement includes that extraction but establishes responsibility boundaries and removes the exception entirely.

Module boundaries

• process.rs — process identity, ownership markers, receipt validation, and termination primitives
• orphan_sweep.rs — same-instance orphan discovery and cleanup
• instance_reaper.rs — foreign/dead desktop instance detection and agent reaping
• lifecycle.rs — tracked runtime synchronization and stale record cleanup
• metadata.rs — model/provider metadata resolution
• runtime.rs — summary/config/spawn orchestration and composition

Validation

At a824fda31eff6ecc0d39ca1b8ea5602a108897e6:

• pre-push desktop-check
• pre-push desktop-test
• pre-push full desktop-tauri-test: 1,637 passed, 0 failed, 14 ignored; integration + doc tests passed
• cargo check --manifest-path desktop/src-tauri/Cargo.toml --lib
• cargo fmt --manifest-path desktop/src-tauri/Cargo.toml --all -- --check
• node desktop/scripts/check-file-sizes.mjs

Supersedes #2974 and #2930.