[BUG] JSON assessment output missing trailing newline

[jwm4]

Bug Description

All JSON assessment output paths produce files without a trailing newline. This violates POSIX conventions and causes `git diff` to show `\ No newline at end of file` on every submission to the leaderboard, as seen in PR #424.

To Reproduce

1. Run `agentready assess `
2. Inspect the generated `.agentready/assessment-*.json`
3. Check: `xxd .agentready/assessment-*.json | tail -1` — the last byte is `7d` (`}`), not `0a` (`\n`)

Expected Behavior

All generated JSON files end with a trailing newline (`\n`), consistent with POSIX conventions and the behavior of tools like `jq`.

Actual Behavior

Generated JSON files end with `}` and no newline. Git diffs show `\ No newline at end of file`.

Affected Code

Four write sites, all missing a trailing newline:

File	Line	Pattern
`src/agentready/reporters/json_reporter.py`	34	`json.dumps(...)` passed to `_write_file()` with no `\n`
`src/agentready/reporters/aggregated_json.py`	38	`json.dump(f)` with no `f.write("\n")` after
`src/agentready/cli/main.py`	324	`json.dump(f)` with no `f.write("\n")` after
`src/agentready/cli/demo.py`	493	`json.dump(f)` with no `f.write("\n")` after

Possible Solution

• In `json_reporter.py`: append `+ "\n"` to the `json.dumps()` result before passing to `_write_file()`
• In the three direct `json.dump()` call sites: add `f.write("\n")` on the line after each dump

The fix should be accompanied by unit tests that assert the written file ends with `\n`.

Environment

• Discovered via leaderboard submission PR Submit assessment for fullsend-ai/fullsend (68.2/100 Silver) #424
• Affects all versions (pre-existing across all write paths)