Add engineering context map and public context index

[danielmarbach]

This adds a small map for people and coding agents working in this repository:

• AGENTS.md: where to look when investigating existing behavior, how to recover a pull request from git log, and when a change deserves an ADR rather than only a pull request description.
• docs/README.md: an index of the design and testing docs we already have under docs/, plus a short list of pull requests that are the canonical record for a decision. There's a rule at the top of that list for what qualifies, so it doesn't turn into "recent PRs".
• docs/decisions/README.md: ADR guidance for the cases where a pull request isn't a good long-term entry point.

Nothing moves, no CI changes, and nothing here adds a requirement for contributors. Pull request descriptions stay the place where single-component decisions get recorded; the map just makes them findable.

I tuned the AGENTS.md wording by running a few agents against real questions about this repo (why ingestion batches are parallel for some persisters, why new RavenDB databases use Lucene, whether the paging change deserved an ADR) and adjusted until they consistently landed on the right source and stopped proposing ADRs for things a pull request already records. The steps about a private context root are there so an agent knows what to do when a person explicitly provides extra context; without that, they are inert.

What I'd appreciate a look at: whether the docs listed in docs/README.md are the ones you'd actually send someone to, and whether the pull request list has the right entries. Feel free to push changes directly; I'm not attached to the wording.

Update: I trimmed docs/README.md to the sources that explain why, after comparing four index variants (no index, full with descriptions, trimmed with descriptions, names only) on five questions, two runs each. The trimmed version with one-clause descriptions reached the right source in the fewest tool calls (2.6 on average versus 3.3 for the full list) and was the only variant with descriptions that never missed. Testing pages stay reachable through docs/testing.md. AGENTS.md also gained two sentences: it changes only when the workflow changes, and how the index is built.

[danielmarbach]

I'm thinking on how we can make the agent file better repo shippable too but that is something we can worry about later in my opinion

[danielmarbach]

Discussed it with Andreas and we concluded having it in master allows the other PRs to be rebased and benefit from it. We can always make more tweaks. Merging