feat(cli)!: replace the one-shot commands with a scriptable surface

The one-shot half of the binary could not finish its headline task without a
human at the keyboard: `search` opened a full-screen picker, panicked with
exit 101 when there was no terminal, and no command could fetch a known file
at all. Everything exited 0 whatever happened, wrote emoji prose to stdout,
and ignored --download-dir, SOULSEEK_DOWNLOAD_DIR and config.toml.

The commands are now built for scripts and agents:

  search  download  get  browse  room list|say|listen  message send|read

stdout carries records only (tab-separated, or NDJSON under --json), stderr
carries progress, and the exit code carries the verdict: 2 usage, 3 connect,
4 no results, 5 timed out, 6 transfer failed. `get` does search, rank and
fetch in one step, and `search --json | jq ... | download --stdin` composes.
Global flags parse in any position and every one has a SOULSEEK_ variable and
a config.toml key resolved through a single precedence chain.

Defects found and fixed along the way, each covered by a test:

- a download with an unknown size was finalized against zero bytes and saved
  as an empty file; the size now comes from the peer's share listing
- transfers are keyed by a hash of the remote path, so the same path from two
  peers shadowed itself and two files sharing a basename overwrote each
  other; both collisions are now dropped with a warning
- `message send` and `room say` slept 500ms and claimed success; they now
  wait for a server reply queued behind the message before reporting
- Client::login waited on an unbounded channel and never returned when the
  server went quiet

Deleted the TUI picker the old `search` drove (download_selector,
MultiDownloadProgress, SearchConfig): 1,573 lines out, 926 in. The
interactive TUI is otherwise untouched.

BREAKING CHANGE: `rooms` and `chat` become `room list|say|listen` and
`message send|read`; `--disable-listener` becomes `--no-listener`; the
unprefixed DISABLE_LISTENER, LISTENER_PORT, LOG_FILE and
MAX_CONCURRENT_DOWNLOADS variables are now SOULSEEK_-prefixed; `search` no
longer opens a picker or downloads anything.

.github/workflows/ci.yml

@@ -98,7 +98,12 @@ jobs:
           test -x soulfind/bin/soulfind
           echo "SOULFIND_BIN=$PWD/soulfind/bin/soulfind" >> "$GITHUB_ENV"
 
-      - name: Run e2e tests (required against soulfind)
+      - name: Run library e2e tests (required against soulfind)
         env:
           SOULSEEK_E2E_REQUIRED: "1"
         run: cargo test -p soulseek-rs-lib --test e2e -- --nocapture
+
+      - name: Run CLI e2e tests (required against soulfind)
+        env:
+          SOULSEEK_E2E_REQUIRED: "1"
+        run: cargo test -p soulseek-rs --test cli_e2e -- --nocapture