fix(cli): reduce over-budget projected list pages instead of shortening values

[ysyneu]

Problem

fduty alert-event list --limit 100 --json (and incident list, incident similar, channel escalate-rule-list) bound their projected structured output to the 16 KiB budget by fair-capping every shortenable string value across every row, marking clipped ones with .... At a large --limit the per-value cap collapses to a few bytes, so the page comes back as 100 rows of x... stubs.

For a --json/--toon consumer this is silent corruption: a jq select(.title | test(...)) or an exact id match runs over the clipped values, misses rows that really match, and an empty result is indistinguishable from a genuine non-match. A stderr note announced the shortening, but the clipped page itself was still the output.

Root cause

boundProjectedList (internal/cli/fieldproject.go) had exactly one tool for an over-budget page: shrink values until all rows fit. It never considered shrinking the page itself — the dimension that costs the consumer nothing, because these commands are already paginated (--limit/--page).

Fix

• On overflow, the page is reduced first: emit the largest leading prefix of rows that fits the budget with every value byte-intact (largestFittingPrefix, a binary search — prefix encoded size is monotone), and announce the reduction on stderr:
  note: emitted N of M projected rows (every value intact) to stay below the 16384-byte structured-output limit; narrow --fields or lower --limit to fit more rows per page — the rows past the first N were not emitted
• Value shortening is kept only for the case page reduction cannot solve: a single row that overflows the budget on its own. Identifier fields stay exempt, every shortened value keeps the ... marker, and the irreducible case (nothing shortenable) still fails with the error naming the largest fields.
• boundProjectedOutput now returns the bounded data (same type as its input) alongside the note; the four list call sites use the returned slice, and the two paginated ones (alert-event list, incident list) pass the emitted count and effective limit to PrintList so its pagination note stays accurate.
• --fields flag help and the alert/incident skill cards now describe the page-reduction semantics.

Notes for reviewers

• Behavior change for --json/--toon list consumers: a large --limit now yields a smaller, fully intact page instead of ...-clipped rows (the point of this change). Scripts that count array length as the page size must heed the stderr notes.
• PrintList's generic raise --limit or use --page note is unchanged (shared helper, wider blast radius); the specific reduction note governs.
• alert list --fields never bounds its projection at all — pre-existing gap, deliberately out of scope.

Verification

$ go test ./internal/cli/ -run 'BoundProjected|AlertEventList|IncidentList|IncidentSimilar|ChannelEscalateRule|FieldsProjection|StructuredFieldsEmpty' -v
... (all pass, including:)
--- PASS: TestAlertEventListAutoReducesPageAtLargeLimit (json, toon)
--- PASS: TestIncidentListAutoReducesPageAtLargeLimit (json, toon)
ok  github.com/flashcatcloud/flashduty-cli/internal/cli

$ make check-cards
skilldoc: cards OK

$ make check   # gofmt/gci, golangci-lint (0 issues), go test -race ./..., build
ok  github.com/flashcatcloud/flashduty-cli/cmd/flashduty
ok  github.com/flashcatcloud/flashduty-cli/internal/cli
... (all packages ok, build succeeds)

The two new AutoReducesPage tests stub 100 rows with ~600-byte mixed CJK titles via the httptest stub and run alert-event list --limit 100 / incident list --limit 100 in both json and toon: exit 0, stdout parses as an array of ≥1 and <100 rows forming the leading prefix of the fixture with every value byte-identical, no ... anywhere, output under 16 KiB, and stderr carries the reduction note naming the emitted count.

TestCommandListProjectionOverflowFails and the end-to-end TestProjectionOverflowFailsHard previously pinned error-on-overflow with multi-row fixtures; those pages now reduce (covered above), so both were re-pointed at the case that still must fail: a single row whose projected non-string field alone exceeds the budget (non-zero exit, empty stdout, error naming the largest fields). Manual eyeball: alert-event list --help shows the new --fields sentence.