Make the CLI drivable without a terminal (#36)

Every parameter of create was answered through an interactive survey picker, so a script or an agent had to allocate a pseudo-terminal and send arrow-key escape sequences. Those sequences are positional offsets into the option lists, so reordering any list silently changed what got selected.

Each picker now has a matching flag and is only reached when its flag is absent. Passing no flags behaves exactly as before, same prompts in the same order. Flag values are validated against the same option lists the pickers offer, so --mins 37 fails loudly instead of becoming an odd chunk count.

Prompting is skipped when stdin is not a terminal, or on –non-interactive, and a missing flag then becomes an immediate error rather than a prompt nobody can answer. archive falls back to archiving on age alone in that case instead of dying on its confirmation prompt.

Also:

• --json writes the created or snoozed task to stdout, so callers no longer have to regex the task id out of a colour-coded logrus line on stderr.
• --due and --notes on create. Due dates were previously unreachable: only snoozeUntil was, via the start prompt, and “review this in a week” and “this is due in a week” are not the same thing.
• snooze takes –id, –title and –until. It was previously picker-only and always snoozed exactly 24h, which remains the –until default. An ambiguous –title lists the candidates rather than guessing.
• –start, –due and –until accept “in hours|days|weeks”, a bare date or an RFC3339 timestamp, not just the four hardcoded presets.
• version prints to stdout, since it is data rather than a diagnostic.

Request bodies are now built with json.Marshal rather than fmt.Sprintf into a JSON string literal. Any quote, backslash or newline in a title produced malformed JSON and an opaque API error, which matters much more once titles are machine-generated. Failed requests also report the response body.

Due and SnoozeUntil are added to Task as pointers, so an unset value is omitted on write rather than sent as the zero time. Verified against all 1781 tasks on the account: every non-null due parses as RFC3339, so the “strange format” comment this replaces no longer holds.

CI built go build main.go, which compiles only that one file; package main is no longer a single file. Switched to ./… and added a test step.