fix: inline --json-schema file paths so structured output works on CLI 2.1.160 (closes #3)

The docs recommend the file-path form (`--json-schema /path/to/schema.json`)
as primary, but the default-pinned Claude CLI 2.1.160 only accepts an inline
JSON string. Passing a path makes the CLI exit 0 with empty output, so the
action's fail-loud branch trips on every run.

Resolve the value before spawning the CLI: a file-path schema is read
in-process and passed inline (validated + compacted), while an already-inline
schema is left untouched. Because the value is handed to spawn() as an argv
element (never through a shell), a schema with `$ref`/`$defs` round-trips
intact. Unreadable files and invalid JSON now fail with a clear error instead
of the CLI's silent empty output.

Docs (README, example workflow, action.yml) are corrected to explain that the
file-path form is auto-inlined and works on the default CLI.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

examples/gitea-structured-output.yml

@@ -2,9 +2,12 @@
 #
 # Instead of having the agent write its verdict to a file, this asks Claude for a
 # result that conforms to a JSON Schema and reads it back from the action output.
-# Requires a Claude CLI new enough to support `--json-schema` (see README →
-# "Structured output"). If you consume this action through a pre-baked runner
-# image (path_to_claude_code_executable), that image's CLI must be bumped too.
+# The default-installed Claude CLI (2.1.160) supports `--json-schema`; the
+# action reads the schema file below and passes it to the CLI inline, so the
+# file-path form works out of the box (see README → "Structured output"). If
+# you consume this action through a pre-baked runner image
+# (path_to_claude_code_executable), that image's CLI must still support the
+# `--json-schema` flag.
 
 name: Claude PR Review (structured output)