[feat] implement backend abstraction, dynamic backend selection, and GPU feature integration

PR_DESCRIPTION.md

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+# Pull Request: PolyScribe workspace + plugin system
+
+This PR refactors the repository into a multi-crate Cargo workspace and adds a minimal, working plugin system scaffold over NDJSON/stdio, while preserving existing CLI behavior. It also introduces a stub plugin `polyscribe-plugin-tubescribe` and documentation updates.
+
+Differences & Adaptations
+- The repository already contained most of the workspace and plugin scaffolding; this PR focuses on completing and verifying the setup, fixing a symlink path issue in the plugin Makefile, and adding documentation and minor cleanup.
+- Existing CLI commands and flags are preserved; a new `plugins` command group is added (list/info/run) without breaking existing outputs.
+
+## Commits
+
+### 1) chore(workspace): scaffold workspace + move crates
+
+Rationale
+- Ensure workspace members and resolver are properly defined. The repository already contained these crates; this commit documents the layout and confirms no absolute paths are used.
+
+Updated files (representative snapshots)
+- Cargo.toml (workspace):
+```
+[workspace]
+members = [
+  "crates/polyscribe-core",
+  "crates/polyscribe-protocol",
+  "crates/polyscribe-host",
+  "crates/polyscribe-cli",
+  "plugins/polyscribe-plugin-tubescribe",
+]
+resolver = "2"
+```
+
+Repository tree after this commit (abridged)
+```
+.
+├── Cargo.toml
+├── crates
+│   ├── polyscribe-cli
+│   ├── polyscribe-core
+│   ├── polyscribe-host
+│   └── polyscribe-protocol
+└── plugins
+    └── polyscribe-plugin-tubescribe
+```
+
+### 2) feat(plugins): host/stdio runner + CLI plugin commands
+
+Rationale
+- Provide plugin discovery and stdio NDJSON JSON-RPC runner in host crate; add `plugins` subcommands to CLI. These were already implemented; this commit verifies and documents behavior.
+
+Updated files (representative snapshots)
+- crates/polyscribe-host/src/lib.rs: discover(), capabilities(), run_method().
+- crates/polyscribe-cli/src/main.rs: `plugins list|info|run` wired to host, forwarding progress.
+
+Repository tree after this commit: unchanged from above.
+
+### 3) feat(plugin): add stub polyscribe-plugin-tubescribe + docs
+
+Rationale (risky change explained)
+- Fixed a symlink path issue in the Makefile by switching from $(PWD) to $(CURDIR) to avoid brittle relative paths. This ensures discovery finds the plugin consistently on all shells.
+- Removed an unused import to keep clippy clean.
+- Added README docs covering workspace layout and verification commands.
+
+Updated files (full contents included in repo):
+- plugins/polyscribe-plugin-tubescribe/Makefile
+- plugins/polyscribe-plugin-tubescribe/src/main.rs
+- README.md (appended Workspace & Plugins section)
+
+Repository tree after this commit (abridged)
+```
+.
+├── Cargo.toml
+├── README.md
+├── crates
+│   ├── polyscribe-cli
+│   ├── polyscribe-core
+│   ├── polyscribe-host
+│   └── polyscribe-protocol
+└── plugins
+    └── polyscribe-plugin-tubescribe
+        ├── Cargo.toml
+        ├── Makefile
+        └── src/main.rs
+```
+
+## Verification commands
+- Build the workspace:
+  - cargo build --workspace --all-targets
+- Show CLI help and plugin subcommands:
+  - cargo run -p polyscribe-cli -- --help
+- Discover plugins (before linking, likely empty):
+  - cargo run -p polyscribe-cli -- plugins list
+- Build and link the stub plugin:
+  - make -C plugins/polyscribe-plugin-tubescribe link
+- Discover again:
+  - cargo run -p polyscribe-cli -- plugins list
+- Show plugin capabilities:
+  - cargo run -p polyscribe-cli -- plugins info tubescribe
+- Run a plugin command and observe progress + JSON result:
+  - cargo run -p polyscribe-cli -- plugins run tubescribe generate_metadata --json '{"input":{"kind":"text","summary":"hello world"}}'
+
+All acceptance checks pass locally.