ecosystem: add the standardized "Part of the Weaver Stack" section + diagram + `weaver-stack` topic

[dgenio]

Why

The Weaver Stack story currently lives mostly in weaver-spec. To make 8 repos read as one coherent project (and cross-pollinate stars), every repo needs the same "Part of the Weaver Stack" block, the same diagram, and a shared discovery topic. This is the ChainWeaver side of weaver-spec#6.

Proposal

• Add a standardized "Part of the Weaver Stack" README section with the shared stack diagram (request path: contextweaver → ChainWeaver → agent-kernel → agentfence; adjacent: vibeguard, lessonweaver, skdr-eval) and one-line "use standalone or together" note.
• Reuse an identical block/wording across all stack repos (coordinate with weaver-spec#6).
• Add the weaver-stack GitHub topic to this repo.

Acceptance criteria

• README contains the standardized stack section + diagram, consistent with the other repos.
• weaver-stack topic added.
• Section explicitly states ChainWeaver works standalone (no hard sibling dependency).

Coordinates with: weaver-spec#6, weaver-spec#5.