feat: allow local log entries to select an issue form #80

Problem

A repository can currently configure one local issue form for every Frog entry. frog log --label can distinguish a class of friction, but the entry is still scaffolded and validated against the repository's single configured form.

Some friction needs structured evidence that does not fit the default form. For example, a flaky-test entry should capture the failing command and output, an unchanged passing retry, environment, and observed frequency.

Proposal

Add a local template override to frog log:

frog log --template flaky-test.yml --label flaky-test "test name: failure"

--template <path-or-name> would resolve the same way as inbound.template: either a full path or a filename under .github/ISSUE_TEMPLATE. When present, it would override the configured or discovered local form for scaffolding and body validation. Existing behavior would remain unchanged when omitted.

This keeps labels and issue forms composable without introducing a broader category or profile abstraction.