fix: config reload concurrency hardening (#3, #4)

[fjacquet]

Fixes two pre-existing config-reload correctness bugs flagged by the Phase 2b review (PR #2), tracked as issues. No public-contract change.

Closes #3
Closes #4

#4 — collector data race

The collector read config.Config.Collect/.Event without Config.Mutex while SIGHUP / --config-watch / /reload mutate them under it — a reader-writer race under -race.

• New config.TakeSnapshot() copies Collect+Event under Config.Mutex into a ConfigSnapshot value (both all-scalar structs → safe deep copy).
• Captured once at the top of Collector.Collect() and stashed on Client.cfg, so the whole scrape sees one consistent config (concurrent same-target scrapes already coalesce). Path discovery (findAllEndpoints, runs at creation) uses its own local snapshot.
• All internal/collector read sites repointed; grep -rn 'config\.Config\.\(Collect\|Event\)' internal/collector/ is now zero in production code.

#3 — watcher silently drops the watch

Atomic editor saves (write→rename) within the 1s dedup window hit the break before the rename/remove re-add, so the inotify watch was dropped and future edits stopped reloading. The handler now does the re-attach first, and the 1s gate suppresses only the ReloadConfig call.

Tests

• internal/collector/config_race_test.go — gathers while a goroutine mutates Collect/Event under the mutex; clean under go test -race (would race on the old code).
• cmd/idrac_exporter/watcher_reattach_test.go — real fsnotify: atomic-save within the window, then a later write still arrives (watch survived). Skips gracefully where the env can't attach a watch.
  • ⚠️ Note: this test exercises the re-attach technique via its own watcher goroutine rather than the unexported WatchConfig loop, so it won't catch a regression of WatchConfig itself. The production reorder is small and reviewed. A follow-up could extract the handler for direct testing.

Validation

• make ci green — go vet, golangci-lint 0 issues, go test -race ./... (56 tests), govulncheck clean
• semgrep clean on changed files

🤖 Generated with Claude Code

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

Run ID: 396ec5d5-0997-4975-839c-338d5f1f269f

📥 Commits

Reviewing files that changed from the base of the PR and between eb292c3 and 0f7166d.

📒 Files selected for processing (7)

• cmd/idrac_exporter/config.go
• cmd/idrac_exporter/watcher_reattach_test.go
• docs/superpowers/specs/2026-06-16-config-reload-hardening-design.md
• internal/collector/client.go
• internal/collector/collector.go
• internal/collector/config_race_test.go
• internal/config/config.go

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix-config-reload-hardening

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