refactor(adapter): let workers own protocol-specific prompt notes (#186)

[Prithvi1994]

Closes #186

Problem

compose_inbound_wakeup_prompt in the host branched on adapter_type == "slack" and rendered Slack guidance from two metadata keys (dmTarget, isActiveThread) that the Slack worker invents. That put protocol-specific behavior in the host and coupled it to the worker through untyped string keys across a Rust/TypeScript boundary — against the rule in exo/docs/tools-and-adapters.md:

The host owns supervision, routing, durable message records, retries, and conversation wakeups. Protocol-specific behavior stays in each worker.

Fix

metadata is already free-form JSON the worker fully controls, so the guidance travels in it:

• Host (crates/executor/src/adapter/runtime.rs): the adapter_type == "slack" block is replaced by a generic loop that appends every string in metadata.promptNotes, in array order, and treats all protocol-specific metadata keys as opaque.
• Slack worker (exo/adapters/slack/worker.ts): emits the same DM-target and active-thread guidance as promptNotes. Wording and order are unchanged, so Slack wakeup prompts are identical to before.
• Docs (exo/docs/design/adapter-architecture.md): the Worker Protocol section documents the metadata.promptNotes contract.

The worker keeps emitting dmTarget / isActiveThread — they are legitimate facts it owns, and dmTarget still builds the note. Only the host's interpretation of them is removed.

Adding another adapter's guidance is now a worker-only change: no host edit, no new if adapter_type == … arm.

Verification

• cargo test -p executor: 117 passed, 0 failed
• cargo fmt --all -- --check and cargo clippy --workspace --all-targets -- -D warnings: clean
• pnpm check (oxfmt, oxlint, tsgo, vitest): passes

New tests:

• wakeup_prompt_appends_worker_prompt_notes_in_order
• wakeup_prompt_ignores_metadata_keys_the_host_does_not_own — regression guard: dmTarget / isActiveThread alone produce no host-authored text
• wakeup_prompt_ignores_non_string_prompt_notes
• wakeup_prompt_tolerates_missing_or_non_array_prompt_notes

Note

No TS unit test for the Slack worker: it performs a live slackAuthTest() at module import, so it is not unit-testable without refactoring (unlike Discord's pure helpers). Typecheck/lint cover the change.

Related, not addressed here

crates/executor/src/adapter/tools.rs still has a Slack-specific dm: target validation branch. #186 lists it as an instance of the same pattern but not the subject of the issue.