docs: expand book metrics pages with fuller explanations

[BraCR10]

Summary

Rewrites book/src/metrics/*.md as narrative prose instead of terse bullet fragments.
Each metric now explains what it measures, how to interpret the number, and the exact
Nostr event kind/tag it's computed from.

Test plan

• mdbook build succeeds with no warnings

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📥 Commits

Reviewing files that changed from the base of the PR and between e1ccb44 and ad8a8ce.

📒 Files selected for processing (7)

• book/src/installation.md
• book/src/metrics/README.md
• book/src/metrics/activity-grid.md
• book/src/metrics/context-signals.md
• book/src/metrics/disputes-bond-policy.md
• book/src/metrics/longevity-liveness.md
• book/src/metrics/trade-size-consistency.md

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