fix: UTF-8 panics + refactor: channel supervisor & routes split

[slysian]

Replace all unsafe byte-index string slicing (&s[..N]) with
str::floor_char_boundary() across 18 files in runtime, kernel, api,
memory, channels, and cli crates.

These cause panics when the byte index falls inside a multi-byte UTF-8
character (common with CJK text from QQ/Telegram users). The crash was
first observed when web_fetch returned Chinese web content that was
truncated at a 3-byte character boundary.

Affected hot paths:

• context_budget: tool result truncation
• context_overflow: overflow recovery truncation
• compactor: conversation text tail-keeping
• stream_chunker: forced break at max_chunk_chars
• web_fetch: HTTP response body truncation
• kernel: session topic & identity file truncation
• docker_sandbox: stdout/stderr truncation
• tool_runner: command/url logging, canvas_id slicing
• provider_health: error body truncation
• subprocess_sandbox: command logging
• session_repair: injection marker stripping (to_lowercase byte mismatch)
• cron/triggers: error message & content truncation
• session (memory): thinking text truncation
• TUI screens: ID/value display truncation

Create supervisor.rs with centralized reconnection/backoff infrastructure
and refactor 23 channel adapters to use it, eliminating ~10,000 lines of
duplicated reconnection logic.

The supervisor module provides:

• SupervisorConfig: configurable initial/max backoff (default 1s/60s)
• run_supervised_loop(): generic supervised reconnection loop
• run_supervised_loop_reset_on_connect(): variant that resets backoff
  after successful connection
• DEFAULT_CHANNEL_BUFFER (256): shared constant replacing hardcoded sizes

Each adapter's inline reconnection loop is extracted into a standalone
async fn that returns Result<bool, String>:

• Ok(true) = reconnect (transient failure)
• Ok(false) = permanent stop (shutdown or channel closed)
• Err(msg) = retry with backoff

Refactored adapters: telegram, discord, slack, irc, mattermost, revolt,
matrix, mastodon, zulip, bluesky, twitch, guilded, gotify, gitter,
discourse, ntfy, nostr, webex, twist, mumble, nextcloud, reddit,
keybase, linkedin.

…ules

Break the single routes.rs god file containing 174 handler functions
into domain-specific modules under routes/:

agents.rs (1,551 lines, 24 handlers)
channels.rs (1,220 lines, 7 handlers)
hands.rs (668 lines, 11 handlers)
models.rs (554 lines, 8 handlers)
skills.rs (489 lines, 9 handlers)
sessions.rs (384 lines, 10 handlers)
common.rs (330 lines, shared types/helpers)

• 22 smaller domain modules

Backward compatibility is maintained via pub use re-exports in
mod.rs, so server.rs and test files continue referencing
routes::handler_name without changes.

Co-Authored-By: Claude Opus 4.6 noreply@anthropic.com