docs(cookbook): revamp semantic-caching against playbook

[SuhaniNagpal7]

What changed

Area	Before	After
Frontmatter	title + description only	Full kebab-case schema: slug, date, author, products, frameworks, difficulty, time-to-complete, tags, og-image, canonical, last-tested-date, structured last-tested-with, code-repo-url, page-type
What you'll build	Missing	5-bullet artifact list before Prerequisites
Why this matters	Generic abstract framing	Reader-facing diagnosis: "your support bot's bill grows with rephrasings, not new questions"; reader can verify the symptom in their own logs. Names the standard tools that miss it (HTTP, Redis, prompt-hash), the FAGI fix (L1 exact + L2 semantic at gateway), and the proof metric (x-agentcc-cache header values)
Install cell	pip install openai agentcc (unused)	pip install openai only
Em dashes	12 occurrences	0
Troubleshooting	3-column table	4 columns with Verify
Next steps	3 reference cards	4-item technical ladder (force-refresh CI hook, per-env namespaces, threshold tuning, fallback routing) + 1 reference link
Step 4 closer	No screenshot	Real Gateway -> Providers -> Cache panel showing 80% hit rate, 30 requests, 24 hits, $0.0089 saved (real UI, mock numbers matching the cookbook's example math)

Why this is shaped the way it is

• The reader-facing "your bill grows with rephrasings" lead is meant to make them check their own request logs before committing to the recipe. Standard caches don't catch paraphrases; this is the failure mode the cookbook fixes.
• The 4-item Next steps ladder is engineering moves, not commercial CTAs: force-refresh in CI, namespaces for per-env isolation, threshold tuning against real logs, routing policy on top of the cache.
• The Step 4 screenshot is the real Cache panel (localhost:8000 dashboard layout), not a from-scratch mockup. Numbers are overlaid so they match the cookbook's worked example (6 unique * 5 = 30 calls; 6 misses + 24 hits = 80% hit rate; $0.0111 baseline - $0.00222 actual = ~$0.0089 saved).

Depends on

• #660 for the <Mermaid> component is not required here. This cookbook has no <Mermaid> usage. Can merge independently.

Test plan

• pnpm dev and visit /docs/cookbook/command-center/semantic-caching -- confirm the new Step 4 screenshot loads from S3 and all 5 step blocks render.
• Em-dash check: grep -nE "(-|-)" src/pages/docs/cookbook/command-center/semantic-caching.mdx returns no hits.
• pnpm astro build passes.

Out of scope

• The notebook companion (cookbooks/command-center/semantic-caching.ipynb) has an equivalent revamp pending in the future-agi/cookbooks repo. Tracked separately; not part of this PR.