fix(cli): keep stdout parseable in JSON mode

[r0h1tb]

Description

JSON is the default output for query, goto and callers — --humanize is
the opt-out — but every failure path bypassed the formatter and printed a Rich
line to stdout instead:

json.loads(subprocess.check_output(["ast-rag", "goto", name]))

works while the symbol exists and raises JSONDecodeError the moment it does
not. _warn_if_ambiguous breaks it from the other side — its docstring says
the note is returned "for JSON consumers", but it also prints the note to
stdout, ahead of the document and wrapped mid-sentence by Rich:

ambiguous: 'main' matched 2 symbols (2 python). Reporting on a.main. Re-run with
a qualified name to pick another, for example b.main
{
  "target": "a.main",
  ...

Diagnostics now go to a stderr console, empty results go through the formatter
so stdout is still a document, and the ambiguity note is carried in the payload
as ambiguous rather than only as a coloured line.

Exit codes are unchanged — 0 and 1 still separate the cases.

Related Issue

Relates to #15 (--format json everywhere). That issue assumes the commands
have no JSON; three of them already default to it. This fixes the paths where
that default is broken. The remaining flag inconsistency — refs and
symbol-impact take --format table|json and default to table, stats
takes --json — is untouched here and worth its own PR.

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature
• Breaking change
• Documentation update

Checklist

• My code follows the code style of this project
• I have added tests that prove my fix works
• All new and existing tests passed
• I have updated the documentation accordingly
• I have run ast-rag evaluate --all — needs live services; no query or scoring path touched
• My changes generate no new warnings

Testing

New tests/test_json_output_is_parseable.py drives the real Typer app through
CliRunner with the API mocked, so no Neo4j or Qdrant is needed, and asserts
json.loads(result.stdout) on every empty, not-found and ambiguous path.

Without the fix, 6 of its 10 tests fail:

FAILED test_goto_not_found_still_emits_json
FAILED test_query_no_results_still_emits_json
FAILED test_callers_symbol_not_found_still_emits_json
FAILED test_callers_with_no_callers_still_emits_json
FAILED test_ambiguous_name_keeps_stdout_parseable
FAILED test_ambiguity_note_survives_into_the_json

Two of the ten guard the other direction — --humanize must still produce
Rich text, and the human path must not go silent now that the "not found"
message moved off stdout.

Suite: 257 passed / 1 skipped / 1 xfailed before, 267 / 1 / 1 after (10 new).
Also checked outside CliRunner that err_console really writes to fd 2 in a
normal process, since CliRunner captures streams differently than a pipe.