fix(channels): add host-based credential injection to WASM channel wrapper (#421)

* fix(channels): add host-based credential injection to WASM channel wrapper

The channel WASM wrapper was missing the host-based credential injection
that the tools wrapper implements. The `credentials` block in channel
capabilities files was dead code: Slack's `on_respond` sends requests
with no Authorization header, expecting the host to inject the bot token
based on `host_patterns`, but the host never did.

This caused Slack (and any channel relying on capabilities-declared
credentials) to fail all outbound API calls with `not_authed`.

Changes:
- Add `ResolvedHostCredential` struct mirroring the tools wrapper
- Add `host_credentials` field to `ChannelStoreData`
- Add `inject_host_credentials()` method on `ChannelStoreData`
- Update `redact_credentials()` to also scrub host-injected secret values
- Add `secrets_store` field to `WasmChannel` + `with_secrets_store()` builder
- Add `resolve_channel_host_credentials()` async helper that decrypts
  capabilities-declared credentials before each WASM callback
- Update `create_store()` and all `call_on_*` / `execute_status` /
  `execute_poll` call sites to pre-resolve and pass host credentials
- Fix leak scan ordering: scan runs on WASM-provided values BEFORE host
  credential injection, preventing false-positive blocks on injected
  Bearer tokens (e.g. xoxb- Slack tokens)
- Make `credential_injector` module pub(crate) so channels can reuse
  `inject_credential` and `host_matches_pattern`

Fixes #389, root cause of #413

Co-Authored-By: Claude Sonnet 4.6 <[email protected]>

* fix(wasm): redact URL-encoded credentials, use url::Url, derive Clone

Address review feedback on PR #421:

1. Security: redact_credentials now scrubs URL-encoded forms of secrets
   in addition to raw values, preventing exfiltration via encoded
   representations in error strings from reqwest
2. Use url::Url::query_pairs_mut() for query parameter injection instead
   of manual string manipulation, improving robustness with malformed URLs
3. Derive Clone on ResolvedHostCredential and simplify the per-tick
   clone in the status repeater loop

Co-Authored-By: Claude Opus 4.6 <[email protected]>

* style: cargo fmt

Co-Authored-By: Claude Opus 4.6 <[email protected]>

---------

Co-authored-by: Sprite <[email protected]>
Co-authored-by: Claude Sonnet 4.6 <[email protected]>

src/tools/wasm/mod.rs

@@ -76,7 +76,7 @@
 mod allowlist;
 mod capabilities;
 mod capabilities_schema;
-mod credential_injector;
+pub(crate) mod credential_injector;
 mod error;
 mod host;
 mod limits;