fix(cli): make incident list --channel match sibling list verbs

[ysyneu]

Problem

incident list was the only list verb whose channel filter used
--channel-id <int64> (a single channel ID). Every other list verb —
alert list, alert-event list, change list — filters by channel via
--channel <comma-separated IDs>. A user or script that learned the
--channel convention from any of those commands and then tried
incident list --channel <id> got unknown flag: --channel.

The API this command calls already supports multiple channel IDs
(ListIncidentsRequest.ChannelIDs []int64), so this was a naming
inconsistency in the CLI, not a capability gap in the backend.

Fix

• incident list now takes --channel <string>, comma-separated channel
  IDs, parsed with the same parseIntSlice helper alert list and
  change list already use, and forwarded as ChannelIDs.
• --channel-id is kept working as a deprecated, hidden single-ID alias
  (cobra's MarkDeprecated, which also hides it from --help) so
  existing scripts that pass --channel-id keep working. --channel
  wins if both are passed.
• internal/skilldoc/build.go's flag walk (used to generate the
  skills/flashduty command reference cards) did not skip hidden flags,
  so the generated card for incident list would have kept documenting
  the now-hidden --channel-id. Added a one-line filter to skip hidden
  flags, matching the existing hidden/deprecated skip that already
  applies one level up, to commands themselves. Regenerated
  skills/flashduty/reference/incident.md via make gen-cards.

Verification

$ go build ./...
$ go test ./...
ok  	github.com/flashcatcloud/flashduty-cli/cmd/flashduty
ok  	github.com/flashcatcloud/flashduty-cli/internal/cli
ok  	github.com/flashcatcloud/flashduty-cli/internal/cmd/cligen
ok  	github.com/flashcatcloud/flashduty-cli/internal/cmd/skilldoc
ok  	github.com/flashcatcloud/flashduty-cli/internal/config
ok  	github.com/flashcatcloud/flashduty-cli/internal/output
ok  	github.com/flashcatcloud/flashduty-cli/internal/skilldoc
ok  	github.com/flashcatcloud/flashduty-cli/internal/timeutil
ok  	github.com/flashcatcloud/flashduty-cli/internal/update

$ go run ./cmd/flashduty incident list --help 2>&1 | grep -- '--channel'
      --channel string    Comma-separated channel IDs

$ go run ./cmd/flashduty incident list --channel-id 1 --help
(exits 0, help renders; stderr: "Flag --channel-id has been deprecated, use --channel instead")

$ make gen-cards && git diff skills/flashduty/reference/incident.md
-- `--channel-id` int64
++ `--channel` string

$ make check-cards
skilldoc: cards OK

$ make fmt && git diff --stat
(no additional diff)

$ make lint
0 issues.