feat(fleet): manage remote Muxa hosts over SSH

[jiunbae]

Summary

Add a production-oriented physical-host Fleet layer to Muxa:

• model host -> session -> window -> pane(agent) with persistent UUID NodeIds
• always publish the controller itself as the first-class local host
• manage Kubernetes-style labels, annotations, selectors, and compact/wide/JSON host views
• connect remote nodes through persistent OpenSSH stdio relays with observe/control policy
• expose Fleet operations through CLI, TUI, local IPC, dashboard API, and MCP
• preserve per-host last-known state and explicit lifecycle/error states
• delegate the single-local-host path to the mature native muxa watch
• document deployment, security, performance, protocol, and operations in English and Korean

This branch contains six logical commits because the local main branch was already one commit ahead of origin/main:

1. 5ea744e adds natural @peer MCP collaboration routing
2. 9c24f8e adds the physical-host Fleet implementation
3. 38f1fba updates h2 for RUSTSEC-2026-0258
4. 7bd8c1b exposes the controller as the always-present local Fleet host
5. b85d426 integrates native watch, compact tables, rich multi-host UI, and push invalidation
6. 964a348 hardens selector subscriptions, navigation, parent messaging, and rendering safety

Local controller node

• available without Fleet configuration and with fleet.enabled = false
• always sorted first as alias local through an in-process adapter
• carries stable NodeId plus immutable local/transport and Kubernetes platform labels
• accepts user labels and annotations through muxa host label local and muxa host annotate local
• supports the same selectors, topology, captures, prompts, attach, dashboard, and MCP surfaces
• cannot be added, removed, disabled, or disconnected
• verifies exact pane keys and rejects a duplicate local NodeId through SSH

Status and watch UX

• terminal-width-aware HOST/STATE/MODE/AGENTS/PANES/ATTN/AGE default status table
• -L, --show-labels, -o wide, and -o json progressively expose richer inventory
• local-only selection delegates to native watch with its complete feature set
• multi-host tree/swarm layouts with host/session/window/pane inspectors and window geometry mosaics
• Arrow Up/Down visits structural rows; j/k jumps directly between actionable panes
• focus/manual/always expansion follows configured view depth without forcing singleton traversal
• Swarm always keeps its visible agent and actual action target synchronized
• m works from a session, window, or pane; parents choose the lowest-index live agent pane
• the shared / message-skill palette works in the multi-host composer
• --include-paneless exposes detached agents, including search and attention filtering, while pane-only actions remain disabled

Architecture, performance, and safety

• no inbound Fleet listener and no SSH port forwarding
• OpenSSH BatchMode stdio transport with protocol/capability negotiation
• observe is the remote default; prompts require explicit control permission
• exact host/backend/session/window/pane identities prevent cross-host collisions
• bounded frames, lines, stderr, pending commands, prompts, and capture payloads
• controller-side terminal sanitization, including remote identifiers and mosaic titles
• no automatic retry for mutating prompt sends
• isolated per-host tasks/caches, bounded connection concurrency, backoff, jitter, keepalive, and command deadlines
• revision-aware snapshots preserve newer state transitions during slow refreshes
• selected-window captures are lazy, rate-limited, and serialized to avoid stale pending races
• selector-scoped fleet_subscribe installs its receiver before ACK; the client fetches a fresh snapshot after ACK
• invalidations coalesce for 75 ms, refresh at most four times per second, and explicitly resync after stream lag
• modern daemons retain a 15-second reconciliation poll; older daemons fall back to one-second polling

User surfaces

• muxa host add|list|show|remove|label|tag|annotate|enable|disable|doctor
• muxa fleet status|watch|connect|disconnect|refresh|panes|capture|send|attach
• muxa watch --fleet --selector ...
• Fleet dashboard read and PAT-protected command endpoints
• muxa_fleet_status, muxa_fleet_capture, and muxa_fleet_send_prompt MCP tools
• docs/FLEET.md and docs/FLEET.ko.md

Validation

• cargo fmt --all -- --check
• cargo clippy --workspace --all-targets --locked -- -D warnings
• cargo test --workspace --locked
• muxa core: 643 passed, 1 environment-dependent rmux smoke test ignored
• sink integration: 7 passed
• muxa CLI: 675 passed, plus 12 e2e tests
• muxad: 58 passed
• selector stream filtering plus enter/leave membership round trip
• focus/manual/view navigation, Swarm selection, parent message routing, paneless search, and sanitization regressions
• git diff --check

Relationship to #65

PR #65 manages agent work inside a local Muxa environment. This PR manages physical nodes and their multiplexer hierarchy. They are complementary; ordinary README/CLI conflict resolution may be needed if #65 lands first.

Rollout

The controller works immediately as local. Controller and remotes should run the same Fleet-capable Muxa version. Start remotes in observe mode, verify muxa host doctor, then grant control only where remote prompt dispatch is needed. Cut a release after review and merge.