Adds shortcuts/mail/flag_suggest.go (~120 LOC) implementing a cobra
FlagErrorFunc hook for the mail subcommand tree. On 'unknown flag: --X'
or 'unknown shorthand flag: "X" in -X', it collects flags from the
current command via cmd.Flags().VisitAll, runs bidirectional prefix
match + Levenshtein DP (threshold=max(1,len/3+1), cap 4), and returns
top-5 candidates inside the existing ErrorEnvelope JSON:
error.type = "unknown_flag"
error.detail.{unknown, command_path, candidates}
error.detail.candidates[*] = {flag, shorthand, distance, reason}
Exit code stays 1 (ExitAPI), not ExitValidation - no breaking change for
CI/agent scripts that check non-zero exit. stderr switches from plain
'Error: unknown flag: --X' to JSON envelope, aligning with the existing
'errors = JSON envelope on stderr' convention; mail unknown-flag was the
last gap.
Scope is strictly the mail subcommand tree: shortcuts/register.go gains
a single 'if service == "mail" { mail.InstallOnMail(svc) }' branch
after the existing Mount loop. Other domains (calendar / im / api /
auth / ...) keep cobra's default FlagErrorFunc and unchanged plain-text
stderr behavior.
Covers:
- shortcuts/mail/flag_suggest.go (new, ~120 LOC)
- shortcuts/mail/flag_suggest_test.go (new, 12 table-driven tests)
- shortcuts/register.go (+3 lines after mail Mount loop)
No changes to cmd/root.go or internal/output/* - ErrDetail.Detail is
already interface{}, handleRootError already routes *ExitError via
WriteErrorEnvelope.
