feat(deckies): generic file drops on fleet + MazeNET deckies

Extracts the docker-exec-with-base64-stdin pattern out of canary/planter
and orchestrator/drivers/ssh into a shared decnet.decky_io package.
Both consumers now delegate; the canary planter test still proves the
contract end-to-end.

Adds POST/DELETE /api/v1/deckies/files for arbitrary file drops.
Container resolution is shared with the canary path: topology_id absent
means fleet (<name>-ssh), present routes through resolve_decky_container
which picks <name>-ssh when the topology decky exposes ssh, else the
topology base container decnet_t_<id8>_<name>.

Path validation rejects relative paths and '..' traversal at the request
model layer.  Bad base64 → 400; unknown topology → 404; decky not in
topology → 422; docker exec failure → 409.

decnet/decky_io/write.py

@@ -0,0 +1,124 @@
+"""``docker exec``-driven file write/delete inside a decky container.
+
+The write path streams a base64-encoded payload over stdin to
+``base64 -d`` inside the container, so binary content of any size up
+to docker's stream limits is safe — interpolating bytes into argv
+would trip ARG_MAX (~128 KB on most kernels) for any non-trivial blob.
+"""
+from __future__ import annotations
+
+import asyncio
+import base64
+import shlex
+from datetime import datetime, timezone
+from typing import Optional
+
+from decnet.logging import get_logger
+
+log = get_logger("decky_io.write")
+
+_DOCKER = "docker"
+_DEFAULT_TIMEOUT = 8.0
+
+
+def _dirname(path: str) -> str:
+    idx = path.rfind("/")
+    if idx <= 0:
+        return "/"
+    return path[:idx]
+
+
+async def _run(
+    argv: list[str],
+    *,
+    stdin_bytes: Optional[bytes] = None,
+    timeout: float = _DEFAULT_TIMEOUT,
+) -> tuple[int, str, str]:
+    try:
+        proc = await asyncio.create_subprocess_exec(
+            *argv,
+            stdin=asyncio.subprocess.PIPE if stdin_bytes is not None else None,
+            stdout=asyncio.subprocess.PIPE,
+            stderr=asyncio.subprocess.PIPE,
+        )
+    except FileNotFoundError as exc:
+        return 127, "", f"argv[0] not found: {exc}"
+    try:
+        stdout, stderr = await asyncio.wait_for(
+            proc.communicate(input=stdin_bytes), timeout=timeout,
+        )
+    except asyncio.TimeoutError:
+        try:
+            proc.kill()
+        except ProcessLookupError:
+            pass
+        return 124, "", "timeout"
+    return (
+        proc.returncode if proc.returncode is not None else -1,
+        stdout.decode("utf-8", "replace"),
+        stderr.decode("utf-8", "replace"),
+    )
+
+
+async def write_file_to_container(
+    container: str,
+    path: str,
+    content: bytes,
+    *,
+    mode: int = 0o644,
+    mtime: Optional[datetime] = None,
+    timeout: float = _DEFAULT_TIMEOUT,
+) -> tuple[bool, Optional[str]]:
+    """Write *content* to *path* inside *container* via ``docker exec``.
+
+    The directory above *path* is created if missing; *mode* is applied
+    after the write; when *mtime* is provided the file is backdated via
+    ``touch -d`` (UTC ISO 8601).
+
+    Returns ``(success, error_or_none)``.  ``error`` is the trimmed
+    docker stderr on rc != 0, or a short "rc=<n>" if stderr was empty.
+    """
+    if not path:
+        return False, "empty path"
+
+    encoded = base64.b64encode(content)
+    parts = [
+        f"mkdir -p {shlex.quote(_dirname(path))}",
+        f"base64 -d > {shlex.quote(path)}",
+        f"chmod {mode:o} {shlex.quote(path)}",
+    ]
+    if mtime is not None:
+        ts = mtime.astimezone(timezone.utc).strftime("%Y-%m-%d %H:%M:%S UTC")
+        parts.append(f"touch -d {shlex.quote(ts)} {shlex.quote(path)}")
+    sh_cmd = " && ".join(parts)
+    argv = [_DOCKER, "exec", "-i", container, "sh", "-c", sh_cmd]
+    rc, _stdout, stderr = await _run(argv, stdin_bytes=encoded, timeout=timeout)
+    success = rc == 0
+    if success:
+        return True, None
+    err = stderr.strip()[:256] or f"rc={rc}"
+    log.warning(
+        "decky_io.write failed container=%s path=%s rc=%d stderr=%r",
+        container, path, rc, stderr[:120],
+    )
+    return False, err
+
+
+async def delete_file_from_container(
+    container: str,
+    path: str,
+    *,
+    timeout: float = _DEFAULT_TIMEOUT,
+) -> tuple[bool, Optional[str]]:
+    """Best-effort ``rm -f`` of *path* inside *container*.
+
+    Returns ``(success, error_or_none)``.  ``rm -f`` returns rc=0 even
+    when the file is already gone, so a True result here means "the
+    file is not present after this call", regardless of who unlinked it.
+    """
+    sh_cmd = f"rm -f {shlex.quote(path)}"
+    argv = [_DOCKER, "exec", container, "sh", "-c", sh_cmd]
+    rc, _stdout, stderr = await _run(argv, timeout=timeout)
+    if rc == 0:
+        return True, None
+    return False, stderr.strip()[:256] or f"rc={rc}"