feat(tokenstore): adopt shared tokenstore with configurable backends

[appleboy]

• Remove the custom file-based locking implementation and its tests.
• Introduce a shared tokenstore dependency and add keyring-related indirect dependencies.
• Replace manual token file read/write logic with tokenstore abstractions for loading and saving tokens.
• Add a configurable token storage backend with support for auto, file, and OS keyring modes.
• Update CLI flags and configuration to select and initialize the token storage backend.
• Refactor tests to use the new tokenstore API instead of direct file and JSON handling.
• Simplify TUI token types by aliasing them to the shared tokenstore token type.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 65.78947% with 13 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
main.go	65.78%	13 Missing ⚠️

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[Copilot]

Pull request overview

Adopts the shared sdk-go/tokenstore package to manage OAuth token persistence, replacing the CLI’s bespoke JSON + file-locking implementation and adding a configurable backend (file vs OS keyring vs auto fallback).

Changes:

• Replace tui.TokenStorage with an alias to tokenstore.Token and remove the local multi-client JSON map types.
• Initialize a configurable tokenstore.Store in initConfig (modes: auto, file, keyring) and wire it into the TUI deps.
• Remove the custom file-lock implementation/tests; refactor token save/load tests to use the new tokenstore API.

Reviewed changes

Copilot reviewed 6 out of 7 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
tui/types.go	Aliases TokenStorage to the shared tokenstore.Token.
main.go	Adds -token-store config and routes token load/save through tokenstore.Store; removes bespoke file token logic.
main_test.go	Updates token persistence tests to use tokenstore.NewFileStore + Save/Load.
go.mod	Adds github.com/go-authgate/sdk-go and keyring-related indirect deps.
go.sum	Records checksums for the newly introduced dependencies.
filelock.go	Deletes the custom cross-process lock implementation.
filelock_test.go	Removes tests for the deleted file-locking implementation.

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