… channels (#5931)

* feat(channels): port command-policy and message coalescing to sidecar channels

Ports two per-channel overrides that the in-process Telegram adapter had
but the sidecar architecture dropped (#5241), so sidecar operators can
replicate them again (closes #5841).

disable_commands / allowed_commands / blocked_commands command policy is
re-exposed on `[[sidecar_channels]]` as `command_policy` (allow / disable
/ allowlist / blocklist) plus `allowed_commands` / `blocked_commands`.
A rejected command is forwarded to the agent as plain text, never
answered with an "unknown command" error — matching the #2063 behaviour
public-facing bots rely on.

message_coalesce_window_ms re-exposes the inbound debounce window (#4441):
messages from one sender arriving within the window are buffered and
dispatched as a single batched context, so a user forwarding several
messages in rapid succession produces one agent invocation instead of
racing concurrent ones on the same session.

Implementation reuses the gating the bridge already has. `SidecarAdapter`
builds a `ChannelOverrides` from its `[[sidecar_channels]]` block and
exposes it via a new `ChannelAdapter::channel_overrides()` default trait
method; the bridge prefers that per-instance override over the
kernel-level channel-type lookup, which cannot tell two sidecars sharing
a `channel_type` (e.g. two Telegram bots) apart. `command_policy` maps
onto the existing `is_command_allowed` boolean/list fields and
`message_coalesce_window_ms` onto the debouncer's `message_debounce_ms`,
so no Python sidecar change is needed — gating happens uniformly
kernel-side for every adapter type.

Tests:
- librefang-types: serde defaults are backward-compatible (allow /
  coalescing-off); disable / allowlist / blocklist parse.
- librefang-channels: `overrides_from_sidecar_config` maps each policy
  variant and the coalesce window correctly, returns None for a plain
  config, and the adapter surfaces the built overrides.

Verified in the sanctioned Docker dev image (named volumes, host target
untouched):
- cargo test -p librefang-types -p librefang-channels --lib (1333 pass)
- cargo clippy -p librefang-types -p librefang-channels --all-targets -- -D warnings (clean)
- cargo fmt -p librefang-types -p librefang-channels -- --check (clean)

The kernel-config golden fixture was updated by hand (the regen test
lives in librefang-api, whose build hits a pre-existing aarch64-Linux
`libc::SYS_getpgrp` break in librefang-runtime unrelated to this change;
the golden assertion is currently `#[ignore]`d on main for separate
pre-existing drift, so CI does not gate on it yet).

* fix(channels): fail closed when allowlist policy has no commands

`command_policy = "allowlist"` is an explicit default-deny intent, but with an empty (omitted or `[]`) `allowed_commands` the sidecar produced `ChannelOverrides { allowed_commands: [] }`.
The bridge's `is_command_allowed` treats an empty allowlist as "no allowlist configured" and falls through to the empty-blocklist branch, which allows everything — so selecting allowlist silently became fail-open, the exact public-bot scenario the policy is meant to lock down.

Map an empty allowlist onto `disable_commands = true` (the highest-precedence deny `is_command_allowed` honours) so it denies all commands.
A non-empty allowlist still allows only its listed commands; `disable` and `blocklist` are unchanged.
An empty blocklist keeps its intuitive "block nothing, allow all" meaning but now logs a WARN since it usually signals a forgotten list.

Tests:
- sidecar unit test asserts allowlist + empty list yields `disable_commands == true`.
- new bridge integration test builds the override through the real `SidecarAdapter` and drives `/agent coder` through the live dispatch path, asserting the command is gated (forwarded to the agent as the plain text `/agent coder`, router unchanged) rather than honoured.

Verified via the repo Docker dev image: `cargo test -p librefang-channels` (479 lib + 28 integration + 4 conformance + 3 doctests, all pass) and `cargo clippy -p librefang-channels --all-targets -- -D warnings` clean.

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Co-authored-by: Evan <[email protected]>