fix(incident): clarify comment length cap unit and report actual count

[ysyneu]

What

flashduty incident comment rejects comment text longer than 1024 characters. Two usability gaps around that cap:

1. Ambiguous unit in help text. The help said "at most 1024 characters" without saying how a character is counted. A script measuring with wc -c (bytes) over-trims multibyte text (e.g. Chinese) that is actually compliant.
2. Over-cap error hid the actual length. The error said only --comment-file content must be at most 1024 characters, leaving the caller to guess how much to trim.

Changes

• Help text now states the limit counts characters (Unicode runes), not bytes — multibyte text is not penalized — and to measure with wc -m, not wc -c.
• The over-cap error now reports actual vs limit, e.g. --comment-file content is 1025 characters, limit is 1024. It still fails before any API call is made.

The counting itself was already rune-based (len([]rune(...))), matching the server-side check; the cap value is unchanged. Only the message and help text changed.

Tests

• Extended the over-cap test to assert the error contains both the actual count and the limit.
• Extended the help-text test to assert the wc -m guidance is present.
• The existing test that a 1024-rune multibyte comment is accepted still passes.
• make fmt, make lint, make test (go test -race ./...), make check-cards, make build all pass.