Add markdown memory.md

[arita37]

Add a markdown memory.md as state saving of learning
each session :

    - Recent call arguments to codebase memory mcp
   - Summary of recent conclusion ....

---> This helps maintain of decision process of the repo, very helpful.

[DeusData]

Thank you, @arita37 🙏 There's real overlap here with the manage_adr tool (which already persists architecture decisions into the graph) and with #490 and #504 (indexing docs and importing ADRs). A free form memory.md of recent calls plus recent conclusions is a different thing from ADRs though, more of a session or working memory log.

We'd lean toward expressing this through the existing ADR and notes mechanism rather than a second parallel state file, to avoid two sources of truth. Could you say more about what you'd put in memory.md that the ADR store wouldn't capture? That'll help us decide whether it's a docs addition or a new lightweight notes API!