Treat concurrent default-group creation as success

[lorenzozanee]

Summary

• Starting several thv processes concurrently could fail with failed to ensure default group exists: group already exists: default when the processes raced the Exists-then-Create window in the default-group startup migration. This is reproducible in the core E2E shard, which runs Ginkgo with PROCS=4.
• The migration now treats groups.ErrGroupAlreadyExists as success: when another process created the default group first, the desired end state already holds. Explicit group create calls (CLI and the API 409 response) still report the conflict.
• --help, -h and help are now classified as informational commands, so printing help no longer triggers migrations or the container runtime check.
• CheckTHVBinaryAvailable now includes stderr in its error so startup failures are diagnosable instead of being masked as thv binary not available.

Fixes #6359

Type of change

• Bug fix

Test plan

• Unit tests (task test)
• Linting (task lint-fix)
• Manual testing (describe below)

New Test_EnsureDefaultGroupExistsConcurrent spawns 8 concurrent default-group migrations against a fresh shared store and requires all of them to succeed (it fails on main with group already exists: default). Manually verified: 10 rounds of 4 concurrent thv processes against a fresh shared XDG state directory all exit 0 (previously 6/10 rounds had at least one process fail), and thv --help on a fresh directory exits 0 without creating the default group or requiring a container runtime.

Does this introduce a user-facing change?

Yes. Concurrent thv startups that previously failed with group already exists: default now succeed, and thv --help no longer requires a container runtime or triggers migrations.

Supersedes #6457 (原PR分支已删，按原提交重建）

[codecov]

Codecov Report

❌ Patch coverage is 80.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 79.08%. Comparing base (f3dfc9e) to head (61e6de7).

Files with missing lines	Patch %	Lines
pkg/migration/migration.go	80.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6668   +/-   ##
=======================================
  Coverage   79.08%   79.08%           
=======================================
  Files         785      785           
  Lines       78381    78385    +4     
=======================================
+ Hits        61989    61993    +4     
  Misses      16387    16387           
  Partials        5        5           

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