docs(providers): define provider-owned batching boundary

[christso]

Summary

• Add the provider invocation boundary to public custom-provider docs for current and v4.42.4 docs.
• Add durable AI-facing guidance in CONCEPTS.md and .agents/product-boundary.md: AgentV dispatches one ProviderRequest per authored provider-case execution; batching, queues, pools, rate limits, fanout, and drain/cleanup are provider-owned adapter internals.
• Reference av-kfik.55 evidence at the agent-guide level without adding public history to user docs.

Validation

• bun install
• bun --filter @agentv/web build
• bun --filter @agentv/web check:routes
• git diff --check
• Red/green docs UAT: git grep -n "Invocation Boundary\|Provider invocation boundary\|runner-selected batch grouping" HEAD -- ... returned no matches; rg found the contract in source and rendered docs HTML.
• Biome note: bunx biome check ... was attempted for the touched Markdown/MDX files, but this repo Biome config processes 0 files for those paths, so it is not applicable.

Coordination

• av-mjgz owns removing old Batch CLI docs that still mention batch_requests.
• av-kfik.56 owns the concrete provider-owned batching example.
• av-di3a owns runtime config/code changes; this PR is docs/guidance only.

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