fix(cli): root usage/error/format hygiene (TASK-2031, BUG-2032)

[xarmian]

What & why

Root-command hygiene for the pad CLI — TASK-2031 + BUG-2032 (PLAN-1985). Two tiny, related newRootCmd()/client edits bundled into one PR.

TASK-2031a — silence the usage dump on runtime errors

SilenceUsage: true on the root command so runtime failures (item not found, network errors) print a clean one-line error with no Cobra usage block. A SetFlagErrorFunc reprints usage for the flag-parse path only (that path genuinely wants it), printing just the usage block and letting Cobra print the single Error: … line — so the message isn't duplicated.

TASK-2031b — echo the failing input on item-not-found

GetItem/UpdateItem/DeleteItem now wrap a bare not_found APIError with the ref + workspace: item TASK-999999 not found in workspace docapp instead of the context-free Item not found. The wrapper (wrapItemNotFound + itemNotFoundError) keeps the clean message and unwraps to the original *APIError so errors.As / structured callers still see Code/Details (Codex round-1 fix). Non-not_found errors pass through unchanged.

BUG-2032a — validate the --format enum

PersistentPreRunE on root rejects any --format outside table|json|markdown instead of silently rendering a table. The help command binds its own local --format (text/md/json/llm), which shadows the persistent flag, so pad help … --format md leaves the persistent formatFlag at its table default and still passes. --version/--help short-circuit before PersistentPreRunE, so they're unaffected.

BUG-2032b — honest markdown advertising

Global --format help changed from output format: table, json, markdown to output format: table, json (markdown on select commands, e.g. item show, project changelog) — markdown is implemented on ~3 of 84 commands.

Files changed

• cmd/pad/main.go — SilenceUsage, SetFlagErrorFunc, PersistentPreRunE, honest --format help
• internal/cli/client.go — wrapItemNotFound + itemNotFoundError (Unwraps to *APIError); GetItem/UpdateItem/DeleteItem wrap not_found
• cmd/pad/format_flag_test.go (new) — --format validation: wiring (negative Execute) + valid/invalid values
• internal/cli/item_notfound_test.go (new) — httptest: not-found wrapping across all three methods, error-chain errors.As assertion, and pass-through of non-not_found errors

Gate results

• go build ./... — PASS
• go vet ./... — PASS
• make lint (golangci-lint v2.11.4) — PASS (0 issues)
• go test ./internal/cli/... ./cmd/pad/... — PASS

Codex review

CLEAN (2 rounds). Round 1 flagged that the not_found wrap discarded the *APIError (losing Code/Details for errors.As); fixed with itemNotFoundError.Unwrap() (a plain %w would have appended : Item not found and broken the clean-message acceptance). Round 2: CLEAN. Confirmed PersistentPreRunE does not break the help command's --format, --version, or --help, and the FlagErrorFunc/SilenceUsage combo is correct.

Empirical before/after (verified against the live server)

command	before	after
item show TASK-999999	Error: Item not found + full usage block	Error: item TASK-999999 not found in workspace docapp (no usage)
item list --format bogus	silently renders a table	Error: invalid --format "bogus": must be one of table, json, markdown (no usage)
item list --bogusflag	error + usage	usage block + Error: unknown flag: --bogusflag (single error, helpful)
help item show --format md	markdown help	markdown help (unchanged — persistent flag not touched)
--version	pad version dev	pad version dev (unchanged)