1.0 polish: structured output and command edge cases (part of #29)

[quinnmil]

Addresses part of #29 (tracking issue). Companion to #35, which handles chart rendering, docs and CI — this branch merges that one in as well, so both land together cleanly (no conflicts between the two on the shared files they both touch).

-n/--limit now applies to --json output too

Every list command applied the limit only to the human table; -n had no effect on --json (or, since TOON's removal, on piped output — the same code path). Introduced limitSlice, called before structuredOutput everywhere: apps list, endpoints list, jobs list, traces list (endpoint and job), errors list, errors occurrences, anomalies list, all four usage modes, and insights list/insights show (which didn't limit in either mode before). Totals and percentages are still computed over the full result set — -n only changes which rows ship.

Other fixes

• scout usage --billing-period --json now includes the billing period and server-reported total (previously identical in shape to a plain --from/--to run).
• jobs metrics --type latency --json no longer carries a total key that's actually execution time, not latency — renamed to execution_time_total to match what it is.
• scout usage --app <id> (without --by-day) now actually filters to that app instead of silently returning all apps.
• scout anomalies --endpoint now accepts the same forms as every other --endpoint flag (Base64 id, plain endpoint name) in addition to the fully-scoped name — previously the base64 form silently matched nothing.
• Empty flag values (--endpoint '', --job '', etc.) now error clearly client-side instead of reaching the server.
• Malformed --job values (including a bare job class name pasted without its queue) now error client-side with guidance instead of a raw API error (500).
• scout auth logout on a fresh install (no config directory yet) is now a no-op success instead of an error.
• --from/--to now reject a reversed range and unreasonable relative durations (negative, or past a 5-year bound).
• --app 0 (or negative) now gives a clear "invalid --app value" error instead of "no app specified."
• scout traces show <job-trace-id> 404s now explain that job traces have no detail endpoint.
• scout errors show omits the Count: line when it's 0 rather than showing a misleading zero.
• scout insights show now prints fields in sorted (deterministic) order, indents nested objects instead of Go's map[...] syntax, and renders nil/empty values as —.

Test plan

• gofmt -l ., go vet ./..., go build, go test -race ./..., golangci-lint run ./... — all clean
• New/updated tests across cmd/root_test.go, cmd/anomalies_test.go, cmd/insights_test.go, cmd/usage_test.go, cmd/traces_test.go, cmd/jobs_test.go, internal/timeutil/parse_test.go, internal/config/config_test.go, internal/api/jobs_test.go
• Live verification against real data: -n under --json for several commands, the anomalies endpoint fix (plain name and full-scoped name both return the same results), a bare job class name rejected cleanly, reversed/negative timeframes rejected on a timeframe-scoped command, --app 0 rejected, auth logout on a fresh config directory

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https://claude.ai/code/session_01BK3fzED9ksTWsvoPfsEwpS

[mitchh456]

This reverts the merged v0.5.0 CHANGELOG entry.

CHANGELOG.md:3 — this branch parents directly on 3aa5f56 ("Bump version to v0.5.0 (#34)"), the commit that added these lines, but the diff replaces ## [0.5.0] - 2026-09-10 and its release summary with ## Pending. On merge the file ends up with two ## Pending headings, no [0.5.0] section at all, and the PR references reverted to the numbers #34 corrected (#23→#8, #24→#18, #22→#20, #21→#19). It's a clean edit on the current file, so git reports MERGEABLE and nothing flags it.

The identical clobber is in #35, so fixing one doesn't fix the other — whichever merges second re-applies it. #32 and #33 branch from the same commit and leave [0.5.0] intact, so this isn't a stale-base artifact.

Is the CHANGELOG restructure intentional? Nothing in the description, the commit message, or #29 mentions it.