fix: standardize User-Agent header across API client, OAuth, and update checks

[carlostasada]

Replace the ad-hoc "alpaca-cli/" string with a shared internal/useragent package that builds APCA-CLI/ /, matching the convention used by Alpaca's other SDKs.

[greptile-apps]

Greptile Summary

Standardizes User-Agent generation across API, OAuth, credential-validation, and update-check requests.

• Adds a shared internal/useragent package that emits APCA-CLI/<version> <os>/<arch>.
• Updates all production HTTP request paths to use the shared format.
• Adds tests for the generated format and updates the API client test coverage.
• Adds .idea to .gitignore.

Confidence Score: 5/5

The change appears safe to merge, with production request paths consistently adopting the shared User-Agent format.

The shared formatter produces a valid platform-qualified value, existing version initialization supplies usable release or development versions, and each changed request path applies the new header without disrupting request construction.

T-Rex Logs

What T-Rex did

• Ran two comparable Go test suites; both exited with exit code 0.
• Performed after-run verification and confirmed that headers matched observed and expected values for cmd.SetVersion, API /v2/account, validation /v2/account, GitHub releases, and OAuth token exchange.
• Retained the generated same-package harness sources to support reproducible proof.

_{Ran code and verified through T-Rex}

Important Files Changed

Filename	Overview
internal/useragent/useragent.go	Introduces the shared platform-aware User-Agent formatter with straightforward deterministic behavior.
internal/client/client.go	Replaces the API client's legacy User-Agent value with the shared formatter.
internal/cmd/root.go	Propagates the configured CLI version into OAuth's newly standardized User-Agent.
internal/cmd/auth.go	Applies the shared User-Agent format to credential-validation requests.
internal/cmd/update.go	Applies the shared User-Agent format to GitHub release checks.
internal/oauth/oauth.go	Initializes OAuth requests with the standardized development User-Agent.
internal/useragent/useragent_test.go	Covers normal and empty-version formatting behavior.

Flowchart

%%{init: {'theme': 'neutral'}}%%
flowchart LR
V[CLI version] --> B[useragent.Build]
B --> F[APCA-CLI/version os/arch]
F --> A[API client requests]
F --> O[OAuth token exchange]
F --> C[Credential validation]
F --> U[Update checks]

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_{Reviews (1): Last reviewed commit: "fix: standardize User-Agent header acros..." | Re-trigger Greptile}