docs(ai): reconcile AI cost docs into one page (supersedes #277)

[maximelb]

Why

Two pages were written for the same feature:

• cost-tracking.md — merged via docs(ai): AI Cost Tracking & Savings page #279; the URL the LimaCharlie web app links to (/9-ai-sessions/cost-tracking/). Covers per-severity valuation, headline metrics, per-profile P&L, CSV exports, and the fleet/MSSP rollup.
• cost-and-savings.md — the open guide in docs(ai): AI Cost & Savings guide #277; stronger narrative (the "who did the work" premise, Mermaid diagrams, a worked numeric example, FAQ) but overlapping, and at a different URL the app doesn't link to.

This folds the best of both into the single canonical page (cost-tracking.md) so there's one cohesive doc, not two.

What was done

• Rewrote docs/9-ai-sessions/cost-tracking.md to combine both:
  • From docs(ai): AI Cost & Savings guide #277: the premise, three Mermaid diagrams (inputs→outputs, case-classification flow, savings pipeline), a fully worked numeric example, logging-human-time, tuning-for-trust, and the FAQ.
  • Kept from the merged page: per-severity valuation, the headline-metrics table (analyst-hours freed, FTE-equivalent, automation rate, cost/investigation), per-profile P&L, cumulative trend, CSV exports, fleet/MSSP rollup, and the per-session max_budget_usd relationship.
  • Reconciled the valuation explanation so the single-profile worked example and the multi-profile per-severity behaviour are both correct and cross-referenced.
• Moved the nav entry up to after User Sessions (the position docs(ai): AI Cost & Savings guide #277 intended), removing the earlier after–API-Reference entry.
• No second page is added; cost-and-savings.md is not introduced.

Supersedes #277 — please close that PR in favour of this one.

Testing

• markdownlint-cli2: 0 errors.
• All internal links verified to resolve to existing .md files; the earlier check-links failure (a bare directory link) is fixed and not reintroduced.
• mkdocs build --strict can't run locally (Material/Mermaid plugin env), so the nav + Mermaid rendering are best confirmed by CI / preview. Mermaid labels are quoted.

🤖 Generated with Claude Code

https://claude.ai/code/session_012ie78qt9UBuvCBEEfLRKT3