feat: support provider target maps

[N4M3Z]

Problem

A provider's target is a single directory, but some harnesses split content by kind — codex reads agents from .codex while its skills belong in .agents. One root per provider cannot express that, so per-kind placement required a second fake provider entry.

Approach

target also accepts a map keyed by content kind with a required default and unknown keys rejected (ProviderTarget::Single | Map). Deploy, drift scoping, release staging, the dashboard, and the TUI resolve through default_target() / target_for_kind().

Test plan

• cargo test --all-features (641 tests, includes new tests/deploy.rs map coverage and provider parsing tests)
• cargo clippy --all-features --all-targets -- -D warnings
• cargo fmt --check

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