fix(incident): rename --channel to --channel-id on incident list

[ysyneu]

What

fduty incident list used --channel, but every sibling channel command uses --channel-id (e.g. channel info --channel-id, channel escalate-rule-list --channel-id). This renames the flag to --channel-id for consistency.

--channel is removed, not aliased — only --channel-id exists on incident list. It filters by channel id and forwards to /incident/list as channel_ids.

Root cause

From prod audit run audit-2026-06-15 (session sess_KtAQ9DLCeiEXVbnTYcfGzw, step 25): an ai-sre agent ran fduty incident list --channel-id <id> and got unknown flag: --channel-id; the working flag was the inconsistently-named --channel. Agents transfer flag knowledge across command groups, so a name that disagrees with the sibling channel commands forces a wasted trial-and-error turn. Precedent: cli PR #41 (incident-list flag doc-vs-code drift) and PR #42 (member list --limit consistency).

Verification

$ env -u GOROOT go build ./...            # clean
$ env -u GOROOT go test ./...             # all packages ok
ok  github.com/flashcatcloud/flashduty-cli/cmd/flashduty   3.667s
ok  github.com/flashcatcloud/flashduty-cli/internal/cli    0.434s
ok  github.com/flashcatcloud/flashduty-cli/internal/config 1.442s
ok  github.com/flashcatcloud/flashduty-cli/internal/output 0.598s
ok  github.com/flashcatcloud/flashduty-cli/internal/timeutil 0.889s
ok  github.com/flashcatcloud/flashduty-cli/internal/update 1.178s

$ gofmt -l internal/cli/incident.go internal/cli/incident_test.go   # clean

# test
--- PASS: TestCommandIncidentListChannelIDFlag   # --channel-id 123 -> channel_ids:[123]

# built binary
$ flashduty incident list --help            # shows: --channel-id int   Filter by channel ID
$ flashduty incident list --channel-id 123  # parses, no "unknown flag"
$ flashduty incident list --channel 123     # Error: unknown flag: --channel   (fully removed)