feat: Add upsert and unique checking in local mode

[matthewmcneely]

Description

This PR adds upsert and unique checking to modusGraph in "local" mode.

Checklist

• Code compiles correctly and linting passes locally
• For all code changes, an entry added to the CHANGELOG.md file describing and linking to
  this PR
• Tests added for new functionality, or regression tests for bug fixes added as applicable

[Copilot]

Pull Request Overview

This PR adds upsert functionality and unique field verification for local (file-based) mode, including improvements to client configuration and documentation support. Key changes include:

• New variadic Upsert method and local-mode support in client.go.
• Additional tests covering upsert, unique constraints, and embedded object handling.
• Documentation updates in README and added configuration options (e.g. maxEdgeTraversal).

Reviewed Changes

Copilot reviewed 10 out of 10 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
upsert_test.go	Added tests for basic and predicate-based upsert operations.
update_test.go	Expanded update tests to cover unique constraints and batch update scenarios.
mutate.go	Introduced unique verification and recursive upsert support, with improved handling of slice and single object.
internal_test.go	Added structural tag tests to ensure proper processing of unique/upsert directives.
insert_test.go	Enhanced insertion tests with embedded objects and depth query adjustments.
client.go	Updated API design with optional upsert predicates, new client options (maxEdgeTraversal), and local-mode logic.
README.md	Revised documentation to reflect local-mode limitations and new upsert features.

Comments suppressed due to low confidence (2)

client.go:258

• [nitpick] Consider renaming the internal upsert helper function (e.g. to 'localUpsert') to clearly distinguish it from the public Upsert method, reducing potential confusion during maintenance.

func (c client) Upsert(ctx context.Context, obj any, predicates ...string) error {

engine.go:82

• Please add a comment explaining the rationale for choosing the value 100000 for TypeFilterUidLimit to aid future maintainers.

worker.Config.TypeFilterUidLimit = 100000