feat: config defaults override poller key/values

[cgrinds]

No description provided.

[Copilot]

Pull request overview

This PR introduces poller-level default configuration application during config loading, by vendoring a struct/map merge helper (Mergo) and using it to merge Defaults into each Poller (only filling zero-valued fields).

Changes:

• Apply cfg.Defaults onto each poller during ReadConfig, while preserving explicitly-set poller values.
• Add unit tests covering default inheritance and nested struct merging.
• Vendor third_party/mergo and update Go indirect dependencies (golang.org/x/net, golang.org/x/sys).

Reviewed changes

Copilot reviewed 9 out of 10 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
config/config.go	Applies defaults to each poller via Poller.applyDefaults() during config load.
config/config_test.go	Adds tests for default inheritance, overrides, nested merges, and integration via ReadConfig.
third_party/mergo/mergo.go	Adds core shared helpers/errors for the vendored merge package.
third_party/mergo/merge.go	Implements deep merge logic used by mergo.Merge.
third_party/mergo/map.go	Implements mergo.Map mapping logic between map/struct.
third_party/mergo/doc.go	Adds package documentation for vendored Mergo.
third_party/mergo/README.md	Adds upstream README for vendored Mergo.
third_party/mergo/LICENSE	Adds upstream BSD 3-Clause license for vendored Mergo.
go.mod	Bumps indirect golang.org/x/net and golang.org/x/sys.
go.sum	Updates sums to match dependency version bumps.

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