feat: add automatic config reload polling

[houyuwushang]

Description

Fixes #904.

Adds opt-in configuration polling through mgmt.auto_reload_interval. The watcher checks file staleness before invoking the existing graceful reload path, so unchanged polls do not increment reload-attempt metrics or engage the reload rate limiter. A successful reload publishes the new configuration back to the watcher, allowing the interval to be changed or disabled without restarting Trickster.

The default remains 0s (disabled), negative intervals are rejected, and the projected-volume test covers the trickster.yaml -> ..data/trickster.yaml symlink swap used by Kubernetes ConfigMaps. The configuration examples now also use the current mgmt section and reload field names.

Validation:

• focused config and auto-reload tests repeated 20-50 times
• race-enabled config, daemon, and reload-handler tests
• go vet ./pkg/config/... ./pkg/daemon and incremental golangci-lint
• Linux daemon-package cross-compilation and vet
• Windows binary build and full example-config validation
• real-process smoke test: a 250ms watcher reloaded a file change, then a reloaded 0s interval prevented a subsequent change from reloading

Type of Change

• Bug fix
• New feature
• Optimization
• Test coverage
• Documentation
• Infrastructure

AI Disclosure

• This contribution DOES NOT include AI-generated changes
• This contribution DOES include AI-generated changes, and I have reviewed the relevant contributing guidelines.

[houyuwushang]

@jranson, could you re-review this when you get a chance? The requested configuration-key and empty-path changes are addressed, and the branch is rebased onto current main.

[jranson]

We're targeting this for the v2.1.0 release, so I'll give it a final review once v2.0.5 release (which includes the new aggregation items from #1068) lands

[houyuwushang]

@jranson v2.0.5 is out now. I rebased this onto current main and resolved the release-line conflicts, so it should be ready for the final review you mentioned.