Fail plan/model/parse on parse errors and surface their diagnostics

[henrygarner]

Fixes #80.

Problem

allium plan returned {"obligations": [], "version": 3} and exit 0 for a spec that does not parse. A consumer could not tell "valid spec with no obligations" from "unparseable garbage".

Root cause: plan, model and parse share run_single_file, which serialised a view of the parsed module and hard-coded ExitCode::SUCCESS, never touching result.diagnostics. Because the parser is error-tolerant (a failed parse still yields a partial module plus diagnostics rather than failing outright), a broken spec produced a well-formed-but-empty result that read as success. The information existed; the single-file path just never wired it to the output or the exit code.

Fix

Every single-file command now routes through a shared single_file_exit:

• Exit non-zero on an error-severity diagnostic (issue resolution 1). Missing-file (exit 1) and wrong-arity (exit 2) are unchanged.
• Merge a diagnostics array into the JSON output, mirroring check (issue resolution 2).
• Warnings and info do not fail the run, so a valid spec with, say, a missing version header still exits 0. This is the distinction the issue drew: check fails on warnings, plan should not.

Help text for all three commands documents the behaviour (issue resolution 3).

Verification (issue matrix)

input	plan exit	diagnostics
unterminated entity	1	error surfaced
total garbage	1	error surfaced
malformed field	1	error surfaced
valid spec	0	[]
valid, warning only (no version header)	0	warning surfaced
missing file	1	— (stderr, no JSON)

Prevention

• The exit-code + diagnostics contract lives in one shared helper, so a new single-file command inherits it by construction.
• cli_smoke regression tests iterate over plan/model/parse, covering the whole family rather than the single reported command.
• docs/project/specs/allium-single-file-tool-behaviour.allium states the contract in Allium, alongside the existing check/analyse specs.

Version bump follows separately on main after merge.