refactor: scale agentic budget by flat per-doc caps

[EricNGOntos]

Summary

• Replace chunk-weighted `per_doc_min_share` with a flat `per_doc_cap` (default 20k) after document selection.
• Grow planning/context pool capacities and ledger/wallet totals with the selected document set so accounting matches spendable budget.
• Deduplicate selected doc IDs when allocating caps.

Test plan

• Agentic retrieve with 1 selected doc: planning/context scale to 1×cap and ledger `total` updates
• Agentic retrieve with N selected docs: planning ≈ N×cap, wallet snapshot/reclaim reflects grown total
• Workflow planner path with `per_doc_cap=0` still behaves
• Confirm classic 3-channel route unchanged when `use_agentic` is not true

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