docs: backfill CHANGELOG and add changelog note to TESTING.md

[AkashGowdaNC]

Closes #45

What this PR does

• Backfills [Unreleased] in CHANGELOG.md with merged milestones (settings YAML, slim deps, Ollama preflight, --skip-preflight, test additions)
• Adds a short "Changelog expectations for tests" section to docs/TESTING.md

Why this is useful

The changelog currently documents only its own creation. This records the work that's already shipped so pre-release history isn't lost.

Changes

• CHANGELOG.md: added missing entries under [Unreleased] → Added and Changed
• docs/TESTING.md: new short section pointing contributors at the changelog policy

How to verify

• python -m pytest tests/ -v passes
• CHANGELOG.md lists the merged milestones
• docs/TESTING.md mentions the changelog policy

[rosspeili]

Thanks for backfilling the history @AkashGowdaNC!

1. Restore ## [Unreleased] header: In CHANGELOG.md, line 9 was removed. The top of the changelog must keep ## [Unreleased] per Keep a Changelog.
2. Deduplicate CHANGELOG.md entries: The file currently has duplicate ### Added and ### Changed blocks at the bottom repeating the doc hub and dependency entries. Please consolidate everything into a single ### Added and ### Changed section under ## [Unreleased].
3. docs/TESTING.md layout: Placing ## Changelog expectations for tests at the very top breaks the document intro. Please move this section to the bottom under a "Contributing & PR Guidelines" heading (or remove it, as CONTRIBUTING.md#changelog-policy and the PR template already cover it).

Once those are cleaned up, this is good to go! <3

[AkashGowdaNC]

Thanks @rosspeili — all three fixed:

• Restored ## [Unreleased] at the top of CHANGELOG.md
• Consolidated the duplicate ### Added / ### Changed blocks into one each under [Unreleased]
• Moved the changelog note in docs/TESTING.md from the top to a new ## Contributing & PR Guidelines section at the bottom

Tests pass. Ready when you are.

[rosspeili]

Almost there @AkashGowdaNC! The CHANGELOG.md consolidation looks great and the new section placement at the bottom of docs/TESTING.md fits the document structure much better.

Just two small cleanup items to fix before merging:

1. Remove accidental duplicate lines in docs/TESTING.md:
   Right after the closing code fence of Step 3 (around line 123), two lines were accidentally pasted outside the code block as raw text right before the new heading:

# Test timestamp presence
assert "timestamp" in t1

Please delete those two dangling lines.

2. Trailing newlines:
   Please ensure both CHANGELOG.md and docs/TESTING.md end with a trailing newline at EOF.

Once those are cleaned up, this is ready to merge!

[AkashGowdaNC]

Thanks @rosspeili — both done:

• Removed the two dangling lines in docs/TESTING.md (they're now only inside the Step 3 code fence)
• Added trailing newlines to CHANGELOG.md and docs/TESTING.md

Ready when you are.