fix: Split clone replace flag on first colon and reject empty search

[ethanstoner]

Bug

jira issue clone --replace "<search>:<replace>" split the value on every colon and required exactly two pieces. Because of that:

• Replacement values that contain a colon (URLs, times like 10:30) were rejected with "Invalid replace string" and the replacement was skipped.
• --replace ":x" got past the check with an empty search string. strings.ReplaceAll(s, "", "x") then put x between every character of the summary and body (the ADF path did the same for each text node).

Root cause

In internal/cmd/issue/clone/clone.go, the replace loop used strings.Split(r, ":") with a len(pieces) != 2 check and never checked for an empty search string.

Change

• Moved the parsing into a small parseReplace helper. It uses strings.Cut(r, ":"), which is the same as strings.SplitN(r, ":", 2), so the value is split on the first colon only. The replacement can contain colons. The search string can't.
• An empty search string is now rejected with the existing "Invalid replace string ... Skipping replacement..." message. The message now also shows the bad value and the reason.
• The --replace flag help now explains the first-colon rule.

Valid <search>:<replace> pairs with no extra colons work the same as before.

Tests

• Added internal/cmd/issue/clone/clone_test.go, a table-driven test for parseReplace (this package had no tests before). It covers a simple pair, an empty replacement, a replacement with colons (URL with a port, a time), a missing separator, an empty search string and empty input.
• I ran the new test against the old split logic first. The colon and empty-search cases failed. They pass with the fix.
• go test ./..., go vet ./... and gofmt -l pass. golangci-lint run ./... reports 0 issues.

Fixes #1029

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