cmd: fix mirror clone help panic

[xhebox]

What problem does this PR solve?

Issue Number: N/A

tiup mirror clone -h panics because the root command skips environment initialization for help requests. Unlike most commands, mirror clone builds part of its help output from the remote mirror index so it needs the TiUP environment even on the help path.

What is changed and how it works?

• Special-case mirror clone -h/--help in the root command so it does not skip environment initialization.
• Keep the existing mirror clone dynamic component flag initialization path intact, so help still includes component flags such as --tidb and --tikv.
• Move the mirror clone initialized marker to after component flags are successfully loaded, so a failed index fetch does not prevent later retries.
• Add a regression test for the root environment-initialization decision.
• Fix branch-wide CI blockers exposed by this PR run:
  • update TiKV worker's UptimeByHost call for the current signature;
  • simplify the Prometheus agent-mode boolean assignment to satisfy the existing cognitive complexity threshold.

Check List

Tests

• Unit test
• Manual test (add detailed scripts or steps below)

Manual test:

go run . mirror clone -h
go run . mirror clone --help

Local validation:

go test ./cmd ./pkg/cluster/spec ./components/playground
make lint
git diff --check

Code changes

• Has exported function/method change
• Has exported variable/fields change
• Has interface methods change
• Has persistent data change

Side effects

• Possible performance regression
• Increased code complexity
• Breaking backward compatibility

Related changes

• Need to cherry-pick to the release branch
• Need to update the documentation

Release notes:

NONE

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[codecov-commenter]

Codecov Report

❌ Patch coverage is 70.58824% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 42.29%. Comparing base (598e27a) to head (2d91390).

Files with missing lines	Patch %	Lines
cmd/root.go	85.71%	1 Missing and 1 partial ⚠️
cmd/mirror.go	0.00%	1 Missing ⚠️
pkg/cluster/spec/monitoring.go	0.00%	1 Missing ⚠️
pkg/cluster/spec/tikv_worker.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2717      +/-   ##
==========================================
+ Coverage   36.22%   42.29%   +6.07%     
==========================================
  Files         329      426      +97     
  Lines       35405    47330   +11925     
==========================================
+ Hits        12823    20015    +7192     
- Misses      20754    24618    +3864     
- Partials     1828     2697     +869     

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