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

[dgenio]

Why

The Weaver Stack story should read consistently across all 8 repos so they feel like one project and cross-pollinate stars. contextweaver already has docs/ecosystem.md, but it needs the same standardized "Part of the Weaver Stack" block + diagram + shared discovery topic as the siblings. This is the contextweaver 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 a one-line "use standalone or together" note.
• Use identical wording/structure across all stack repos (coordinate with weaver-spec#6).
• Add the weaver-stack GitHub topic.

Acceptance criteria

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

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