docs: refresh sidecar references after externalBin → resource move

Commit 025fa19 moved the claude-agent sidecar from Tauri's externalBin
to its resources mechanism, but several docs still described the old
externalBin layout and bundle path. Refresh:

- src-tauri/src/agent_provider/claude/sidecar_path.rs module doc:
  point at `resources` and note why the move happened.
- scripts/build-claude-sidecar.sh header comment: same.
- docs/features/agent-chat.md "How it's shipped": describe the actual
  resource glob, the resource_dir pinning in setup(), and the on-disk
  install paths (`usr/lib/codemux/binaries/` on Linux, `binaries/`
  next to codemux.exe on Windows). Add a packagers note so downstream
  repackagers (AUR, custom distros) know to copy the sidecar from the
  resource layout, not just `usr/bin/codemux`.

No functional change.

Author: Zeus-Deus

docs/features/agent-chat.md

@@ -138,19 +138,34 @@ bun run build:all
 
 The per-target binary is staged at
 `src-tauri/binaries/codemux-claude-sidecar-<target-triple>`, which is
-where Tauri's `externalBin` bundling picks it up. The Rust integration
+where Tauri's `resources` glob picks it up. The Rust integration
 tests (`src-tauri/tests/sidecar_ping.rs`) look for the same path and
 skip cleanly (with a build hint) if the binary is missing.
 
-**How it's shipped.** `tauri.conf.json`'s `bundle.externalBin` array
-includes `binaries/codemux-claude-sidecar`. Tauri's packager picks up
-`binaries/codemux-claude-sidecar-<triple>` per target and embeds it
-into the AppImage / deb / rpm / NSIS installer. The release workflow
-(`.github/workflows/release.yml`) installs Bun and pre-stages the
-binary so tauri-action finds it before bundling. CI
+**How it's shipped.** `tauri.conf.json`'s `bundle.resources` array
+includes `binaries/codemux-claude-sidecar-*`. Tauri's packager picks
+up the per-triple variant and embeds it into the AppImage / deb / rpm
+/ NSIS installer under `usr/lib/codemux/binaries/` on Linux and next
+to `codemux.exe` under `binaries/` on Windows. (It originally shipped
+as an `externalBin` under `usr/bin/`; moved to a resource because
+linuxdeploy's patchelf step corrupts the ~100 MB bun-compiled binary
+during AppImage bundling — see commit 025fa19.) At runtime, the
+`setup()` hook in `src-tauri/src/lib.rs` resolves the resource via
+`AppHandle::path().resource_dir()` and pins the resolved path into
+the `CODEMUX_CLAUDE_SIDECAR_PATH` env var so the adapter (which has
+no `AppHandle` access at construction time) can find it. The release
+workflow (`.github/workflows/release.yml`) installs Bun and
+pre-stages the binary so tauri-action finds it before bundling. CI
 (`.github/workflows/ci.yml`) does the same, with a zero-byte
 placeholder fallback for constrained runners.
 
+**Packagers note.** Downstream packagers (AUR, custom distros) MUST
+copy the sidecar from the Tauri resource layout (`usr/lib/codemux/
+binaries/codemux-claude-sidecar-<triple>` on Linux) into their own
+package — repackaging only `usr/bin/codemux` will leave the agent-
+chat Claude provider unable to find its sidecar and the first send
+fails with `provider_not_configured: Claude`.
+
 **Protocol.** Newline-delimited JSON-RPC 2.0 over stdin/stdout. The
 Rust side spawns the sidecar through
 [`JsonRpcChild`](../../src-tauri/src/json_rpc_child/mod.rs) — the same

scripts/build-claude-sidecar.sh

@@ -1,7 +1,10 @@
 #!/bin/bash
 # Build the claude-agent sidecar for the current target and stage the
 # compiled binary at src-tauri/binaries/codemux-claude-sidecar-<triple>
-# so Tauri's `externalBin` can bundle it into the installer / AppImage.
+# so Tauri's `resources` glob can bundle it into the installer / AppImage.
+# (The sidecar used to ship as an externalBin; it moved to resources
+# because linuxdeploy's patchelf corrupts the bun-compiled binary —
+# see commit 025fa19.)
 #
 # Called by scripts/copy-sidecars.sh (which wraps every sidecar we
 # bundle), by the release workflow, and optionally during `cargo test`

src-tauri/src/agent_provider/claude/sidecar_path.rs

@@ -2,9 +2,11 @@
 //!
 //! The sidecar is a Bun-compiled TypeScript binary staged under
 //! `src-tauri/binaries/codemux-claude-sidecar-<triple>[.exe]` in dev
-//! builds and bundled into the installer via Tauri's `externalBin`
-//! mechanism in release builds. This module finds whichever copy is
-//! reachable from the current process.
+//! builds and bundled into the installer via Tauri's `resources`
+//! mechanism in release builds. (Originally shipped via `externalBin`,
+//! moved to `resources` because linuxdeploy's patchelf step corrupts
+//! the ~100 MB bun-compiled binary — see commit 025fa19.) This module
+//! finds whichever copy is reachable from the current process.
 //!
 //! Search order:
 //!   1. `CODEMUX_CLAUDE_SIDECAR_PATH` env var (tests and manual