feat(redact-text): declare config_schema and consume typed __config (template for #147)

[JordanTheJet]

Summary

Template migration for #147: the canonical reference plugin (redact-text) gains its config_schema and consumes typed __config, ahead of the host enforcement landing in zeroclaw-labs/zeroclaw#9126. The other 30 packages should copy this PR's shape.

• Schema: closed Draft 2020-12, all-optional (replacement string minLength=1, redact_emails boolean, patterns array of non-empty strings), additionalProperties = false. Validated against the real host validator (validate_manifest_config / resolve_plugin_config from the zeroclaw#9126 head) with per-constraint rejection probes and a withheld-grant {} acceptance check; an un-migrated manifest fails the same harness, so the check is not vacuous.
• Guest change is REQUIRED, and skipping it is a hard break: with __config typed, the old HashMap<String, String> field fails the entire ExecuteArgs parse and every call returns "invalid arguments". RedactConfig now derives Deserialize with serde defaults; from_json replaces from_section; JSON null (host injected nothing) yields defaults; malformed config errors, never silently defaults.
• Config errors cannot leak config values: from_json returns a pre-sanitized error string because serde_json::Error's Display quotes the offending value, and this plugin's patterns are exactly the strings an operator wants scrubbed. Mutation-checked: the naive version leaks the value verbatim into the ToolResult.
• Version 0.3.0 in three files (manifest.toml, Cargo.toml, Cargo.lock — CI runs --locked). registry.json untouched; build-registry.py --check-metadata classifies the new version as pending unpublished source, and release CI publishes it.
• README updated from the now-false comma-separated patterns documentation to the JSON-array encoding and instance-key flow.

Checks

cargo test --locked                               14 passed (baseline 7)
cargo clippy --locked --all-targets -- -D warnings clean (host + wasm32-wasip2)
cargo build --locked --target wasm32-wasip2 --release   ok (186,999 bytes)
cargo fmt --check                                 clean
tools/build-registry.py --check-metadata          11 matched + redact-text@0.3.0 pending
tools/ci tests                                    36 OK on a pristine tree

Mutation evidence: six guards disabled individually, each failing exactly its covering test (including the leak guard and the empty-pattern filter, whose removal rewrites entire inputs into replacement markers); all files restored byte-identical.

Template notes for the remaining 30 (also posted to #147)

1. Version bump is three files; the lockfile is the one that gets forgotten.
2. Assume a guest change is needed; grep for HashMap<String, String>, eq_ignore_ascii_case, .split(',').
3. Never let a config error echo the config value.
4. Keep guest-side normalization even where the schema duplicates it.
5. Decide required per key deliberately: credential-bearing channel plugins should fail closed, unlike this one.
6. Do not add deny_unknown_fields guest-side.