feat: Make CLI output machine-readable and pipeable (#588)

Replace ad hoc console.log calls with an injectable Output so the CLI has
one place that decides what goes where, and so output can be asserted in
tests with an in-memory implementation.

- Only command results go to stdout: prompts, spinners, progress, and
  other information are written to stderr.
- Add --json to read request params from stdin and write the response as
  JSON. The JSON format is used automatically when stdout is not a
  terminal, and --no-json opts out.
- Support unix pipes end to end: params may be piped or redirected in,
  and the response piped or redirected out.
- Trim responses to the response key and pagination, using the response
  key from the API blueprint.
- Exit non-zero on a failed request or an error.


Claude-Session: https://claude.ai/code/session_01NKsb8oTieozDEGJtiwWqBo

Co-authored-by: Claude <noreply@anthropic.com>

Author: razor-x

README.md

@@ -97,6 +97,64 @@ seam access-codes create --code "1234" --name "My Code"
 seam access-codes list --device-id $MY_DOOR
 ```
 
+### Output
+
+Only the response is written to stdout, so any command may be piped or
+redirected. Prompts, progress, and other information are written to stderr.
+
+The response is trimmed to the response key and pagination: no other top level
+fields are reported.
+
+```bash
+# The response, and nothing else, ends up in the file
+seam devices list > devices.json
+
+# Prompts and progress still show up in the terminal
+seam devices list | jq '.devices[].device_id'
+```
+
+### JSON
+
+Request params may be piped or redirected in as a JSON object. Params given as
+arguments win over params read from stdin.
+
+```bash
+# Read params from a file
+seam locks unlock-door < params.json
+
+# Or from another program
+echo '{"device_id": "'"$MY_DOOR"'"}' | seam locks unlock-door
+
+# --device-id wins over any device_id in params.json
+seam devices list --limit 5 < params.json
+```
+
+Pass `--json` to write the response as JSON. It is enabled automatically
+whenever stdout is not a terminal, so piping and redirecting produce JSON
+without passing anything. Pass `--no-json` to opt out and get the pretty
+format instead.
+
+```bash
+# Both write JSON
+seam devices list --json
+seam devices list | jq
+
+# Pretty printed, even though it is piped
+seam devices list --no-json | less
+```
+
+Without a terminal to prompt on, the CLI behaves as though
+`--non-interactive` was given: rather than waiting for an answer nobody can
+give, it exits with an error naming what is missing.
+
+```bash
+$ echo '{}' | seam locks unlock-door
+Missing required parameter for /locks/unlock_door: --device-id
+```
+
+An error exits non-zero. A request that fails reports its `error` on stdout,
+so it can be inspected from a pipe; anything else is written to stderr only.
+
 ## Development and Testing
 
 ### Quickstart