fix: omit redundant apply instructions from apply comments

[m-agahi]

🧠 Ai UsageDetails (if applicable):

This PR was written primarily by Claude Code, reviewed and verified by the contributor.

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Summary

Fixes #2120

The PR comment "Instructions" section always showed "To apply these changes, run digger apply" even after an apply had already completed — which doesn't make sense.

This change makes formatExampleCommands() command-type-aware: when the batch/job type is apply, the apply instructions are omitted. The unlock instructions are always shown since they remain useful regardless of command type.

Changes

• libs/comment_utils/summary/updater.go — formatExampleCommands now accepts a DiggerCommand parameter and skips apply instructions when command is apply
• backend/utils/comment_utils.go — same change for the backend's copy of formatExampleCommands
• libs/comment_utils/summary/updater_test.go (new) — tests using a mock PullRequestService to verify the full UpdateComment flow for both plan and apply, plus unit tests for the formatter

Test plan

• go test -v ./libs/comment_utils/summary/ — all 4 tests pass
• go build ./backend/utils/ — backend compiles cleanly
• Verify in a demo repo that plan comments still show apply instructions
• Verify in a demo repo that apply comments no longer show apply instructions

[m-agahi]

no review?