Set provider import policy defaults

[luca-ctx]

Summary

• define the default native provider import policy for local ctx
• remove Codex-specific rich/search/output mode knobs from the product path
• index real conversation text, failed diagnostics, and file-touch metadata while excluding raw diffs and successful tool output
• add policy regressions for Codex, Claude, CodeBuddy, Continue, OpenClaw, ForgeCode, Kimi, Trae, SQLite adapters, and generic native gating
• document storage families and provider fixture expectations

Validation

• cargo fmt --check
• git diff --check && git diff --cached --check
• bash scripts/check-docs.sh
• cargo test -p ctx docs
• cargo test -p ctx-history-capture
• cargo test -p ctx --test native_providers -- --nocapture
• cargo test -p ctx-history-store
• private provider conformance/parity/fuzz checks against this branch