Harden accounts CLI: markup escaping, accountId validation, selections lock

[StuartMeeks]

Summary

Six fixes from a recent code review of 0.6.0, bundled into 0.6.1.

• Escape user/provider-controlled strings in every accounts command (provider names, account names, environments, summary DisplayFields, exception messages, the --provider flag echo). Closes a markup-injection vector where a credential name or JsonException/IO message containing [ or ] garbled the table or error line.
• Validate accountId is a GUID before any filesystem call on Delete / Select / GetCredentialByIdAsync (file backend + Keychain + libsecret). Non-GUIDs resolve to deterministic false on the mutating paths and ArgumentException on the strict read API. Closes a path-traversal / glob-injection surface where * or ..\..\foo could have flowed into Path.Combine / Directory.GetFiles.
• CommandFormatting.ShortId safely abbreviates ids so accounts delete abc (or any short user-supplied id) no longer crashes on the [..8] slice in the confirm prompt before reaching "not found".
• Cross-process lock around selections.json RMW via a new SelectionsLock helper (FileShare.None + FileOptions.DeleteOnClose + ~5s exponential-backoff retry). Atomic rename was already preventing torn files; this closes the lost-update window where two CLI invocations modifying different providers could lose one provider's selection. Keychain/libsecret backends are unaffected — both OS stores serialise their own writes.
• PackageOutputPath was hard-coded as C:\nuget-local\, which on Linux produced surprising src/.../C:/nuget-local/… repo churn. Now repo-relative under artifacts/packages (already gitignored).
• GeneratePackageOnBuild now scoped to Release builds — dotnet test and Debug builds no longer pack.

Drive-by fix: the unbalanced parenthesis in the accounts select / delete choice prompts ({c.AccountId[..8]}... was missing its closing )) — surfaced while replacing the unsafe [..8] slices.

Test plan

• dotnet build clean (0 warnings, TreatWarningsAsErrors on)
• dotnet test — full suite green (145 tests, +18 new)
  • CommandFormatting.ShortId across full GUIDs, short strings, exact-8 boundary, null/empty
  • Non-GUID accountId resolves to deterministic not-found on file-backend Delete/Select; throws on GetCredentialByIdAsync
  • Concurrent SelectCredentialAsync on different providers both persist (regression test for the selections.json race)
  • SelectionsLock uncontended / contended / post-release semantics
• CI green on ubuntu-latest (libsecret) and macos-latest (Keychain)

Notes

• No public API surface change — this is additive hardening on top of 0.6.0.
• CHANGELOG entry added under [0.6.1] — 2026-05-03.