fix(reranker): default RERANKING_STRATEGY to slice, top_n=3

MR 1: fix(reranker) — default RERANKING_STRATEGY to slice, top_n=3

RERANKING_STRATEGY=adaptive returned 0 documents despite 20 chunks retrieved. The adaptive confusion_cost = (1 - score) dominates when TEI scores cap at ~0.46 → marginal value always negative → all chunks rejected.

Var	Before	After
RERANKING_STRATEGY	adaptive	slice
RERANKER_TOP_N	2	3
RERANKING_THRESHOLD	0.5	0.75

Files: docker-compose.yaml, env, architecture.md

MR 2: chore(ansible) — remove el-salvador deployment from main

The El Salvador deployment (deploy/ansible/group_vars/itu_rtx_el_salvador/, inventory/itu_rtx_el_salvador.ini) is specific to the release/el-salvador branch. It belongs there, not on main.

⚠ Rebase note: when rebasing release/el-salvador onto main, KEEP these files (git may try to delete them since main removes them).

Files deleted: vars.yml, vault.yml, itu_rtx_el_salvador.ini