Rename crates/agent to crates/tracing, flux to collector

[renardeinside]

Summary

• Rename crates/agent/ → crates/tracing/ (cargo package apx-agent → apx-tracing), freeing the agent name for the future AI agent surface (🤖 [FEAT] agent surface #130)
• Rename all internal flux/Flux identifiers → collector/Collector to avoid collision with the tracing crate
• Rename CLI subcommand apx flux start/stop → apx tracing start/stop
• Binary name apx-agent stays unchanged — no migration needed for deployed daemons

Test plan

• cargo check passes
• cargo clippy --all-targets — zero warnings
• cargo test --lib — 374 tests, 0 failures
• cargo fmt — clean
• apx tracing start / apx tracing stop — manual smoke test
• apx --help shows tracing instead of flux

Closes #130

🤖 Generated with Claude Code