chore: add cmd aliases

[mbevc1]

• Following customer request adding PR commands aliases to match attestation records.
• Use same file name patterns

Checklist

• Docs are autogenerated from CLI help — make sure the help text reflects your changes
• Helm chart (charts/k8s-reporter/) updated, if needed. Note: these changes live in a separate PR
• Terraform provider and related changes (terraform-provider-kosli, terraform-aws-evidence-reporter, terraform-aws-kosli-reporter) updated, if needed

[claude]

Claude finished @mbevc1's task in 1m 35s —— View job

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Review: PR #1066 — cmd aliases

• Gather context (diff, related commands)
• Static analysis (go build, go vet — both pass ✅)
• Review changes
• Post feedback

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This is a clean, low-risk change. It adds pull_request / merge_request aliases (snake_case, matching the attestation-record type naming) to both the attest pullrequest and assert pullrequest parent commands, and renames assertPullrequest.go → assertPR.go to match the attestPR.go file-name pattern. 👍

What I verified

• ✅ go build ./... and go vet ./cmd/kosli/ pass.
• ✅ Both alias lists are now identical (pr, pull_request, mr, mergerequest, merge_request) — before this PR they differed in order/content, so this is a nice consistency win.
• ✅ The new pull_request alias lines up with the existing type naming used elsewhere (root.go:279 help text lists pull_request as an attestation type; pullrequest.go:31 posts to the .../pull_request endpoint).
• ✅ No autogenerated docs under cmd/kosli/testdata/output/docs/ reference these aliases, so nothing needs regenerating there. Worth a quick make docs (or your doc-gen target) locally just to confirm the published help/docs pick up the new aliases, per the PR checklist.

Minor observations (non-blocking)

• No test asserts the alias set, so a typo or accidental removal wouldn't be caught. Optional, but a tiny table test (e.g. assert newAttestPRCmd/newAssertPRCmd contains each expected alias) would lock this in cheaply. Given how trivial the change is, this is genuinely optional.
• Cobra treats aliases as an unordered lookup, so ordering is purely cosmetic — no functional concern. Grouping them logically (pr, pull_request then mr, mergerequest, merge_request) reads well.

Nit: consider mirroring this snake_case alias addition on any related PR subcommands if customers expect it consistently — but the parent-command aliases resolve regardless of subcommand, so this is fine as-is.

No blocking issues — LGTM. ✅