fix(daemon): surface self-eviction blocker in logs (#51)

[zackees]

Summary

• Add DaemonContext::busy_reason() returning a human-readable reason ("build/deploy operation in progress", "N open serial sessions", "N pending serial attaches") or None when idle.
• Promote self-eviction diagnostics from debug! to info! so users see: (a) when the daemon goes idle and the countdown starts, (b) what's keeping it alive when it can't evict, with a 60s periodic reminder so stuck sessions are visible without tick-by-tick log spam.
• Tests cover empty/busy states and multi-blocker joining.

Context

Partially addresses #51: the reporter saw fbuild-daemon.exe staying resident after autoresearch finished, but the existing self-eviction logic (empty for 120 s → shutdown) logs only at debug level. That made the behavior invisible and indistinguishable from "eviction is broken." This PR surfaces the blocker at info so the next time someone hits this, the daemon log says exactly what's keeping it alive (e.g., Daemon staying alive: 1 pending serial attach). The underlying eviction logic is unchanged.

Test plan

• uv run cargo clippy --workspace --all-targets -- -D warnings clean
• uv run cargo test -p fbuild-daemon --lib — 88 passed
• CI green on Linux / macOS / Windows

🤖 Generated with Claude Code

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Run ID: b2e952eb-5837-483b-882d-75320e440ddc

📥 Commits

Reviewing files that changed from the base of the PR and between 0f01103 and a14461a.

📒 Files selected for processing (2)

• crates/fbuild-daemon/src/context.rs
• crates/fbuild-daemon/src/main.rs

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/issue-51-daemon-eviction-diagnostics

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