feat(assets-sync): warn on unmatched config rules; adopt stderr-persistent plugin output

[lwshang]

Summary

• Warn on unmatched config rules (the original SDK-2714 deliverable): .ic-assets.json[5] rules whose glob never matched any asset are reported to the user, mirroring ic-asset's get_unused_configs().
• Migrate the sync plugin to the new stdout/stderr contract introduced by dfinity/icp-cli#556: the plugin's stderr is now treated as persistent output displayed after the sync step completes, stdout as transient progress.
  • WIT change: exec returns result<_, string> — the option<string> summary payload is gone.
  • Plugin eprintln!s its end-of-run summary; warnings from assets-sync (e.g. unmatched-config-rule patterns) are eprintln!ed inline at the point of detection.
  • plugin/wit/sync-plugin.wit is now byte-identical to the copy in icp-cli.

Release coordination

This PR requires dfinity/icp-cli#556.

CI on this repo installs the latest released icp-cli for the integration tests, so the expected order is:

1. Land feat(sync-plugin): persist plugin stderr output after a sync step icp-cli#556.
2. Cut a new icp-cli release that includes it.
3. Re-run CI here — the integration tests will then resolve against the new release and pass.

Until that release is out, CI on this PR is expected to fail at the integration-test stage.

Test plan

• cargo check --workspace --all-targets is clean
• cargo test -p assets-sync — 71 unit tests pass (3 scan-level warning-Vec tests replaced by focused get_unused_configs tests in config.rs)
• After the new icp-cli release is out: re-run CI here and confirm integration tests pass
• End-to-end run against the new icp-cli: deploy a project containing an .ic-assets.json rule with a typo'd pattern, verify the warning shows up under the canister name after the step completes
• End-to-end: a successful sync prints the summary line persistently (not lost to the rolling buffer)

🤖 Generated with Claude Code

[Copilot]

Pull request overview

This PR completes the SDK-2714 deliverable by adding "unmatched config rule" warnings, and migrates the sync plugin to the new icp-cli stdout/stderr contract (where stderr is persisted past the rolling progress view). The WIT export exec no longer returns an optional summary; the plugin now emits its summary and warnings via eprintln!.

Changes:

• Add AssetSourceDirectoryConfiguration::get_unused_configs() and is_own_for_dir(), capturing the original glob pattern on each rule, and emit per-directory unmatched-rule warnings to stderr from scan::walk.
• Update WIT signature exec from result<option<string>, string> to result<_, string> and have the plugin eprintln! the summary instead of returning it.
• Update plugin docs/WIT comments to describe the new stdout-transient / stderr-persistent contract, and tick the README checklist item.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
plugin/wit/sync-plugin.wit	Document new stdout/stderr semantics; drop optional summary payload from exec return type.
plugin/src/lib.rs	Switch Guest::exec to Result<(), String> and route the summary through eprintln!.
plugin/README.md	Mark the "warn on unmatched config rules" checklist item done.
assets-sync/src/config.rs	Store rule's original pattern, expose get_unused_configs / is_own_for_dir, drop dead_code allow on origin, add focused unit tests.
assets-sync/src/scan.rs	After each owned directory walk, eprintln! warnings from get_unused_configs; replace removed Vec-based test with a smoke test; introduce scan_sources alias.

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