docs: RELEASE-NOTES.md for v1.3.0

Replaces the v1.2.0 notes, as each release does.

Every figure is measured rather than recalled: 48 suites and 854 cases, counted
the way scripts/draft-release-notes.mjs counts them (v1.2.0 that way is 40 and
599, which is what its notes say); the per-suite counts from each suite's own
last line; the context windows by running both releases' catalog over the model
ids; the token lifetimes from the backend's EditorToken; the plan each model
starts at from the live pricing endpoint.

The draft script itself could not be used: it refuses to run once the tag
exists, and v1.3.0 was tagged first. So these notes land after the tag, as
v1.2.0's did.

Described relative to v1.2.0, not to this cycle's own review rounds — a state
that only ever existed between two commits here is not something a user saw.

One claim is flagged in the text as unproven on screen: the agent's browser
preview. The chain is tested up to the call that opens the tab.

Author: ndemianc