(remote): show a remote host's capability tier and disable what it cannot do (#218)

[devsuitup]

First slice of #218 (capability tiers and the host profile).

What changed

• remote-host-profile.js: a pure computeHostProfile({ at, error, descriptors }) returning the highest tier reached among observe, liveness, inject, attach, launch, plus the reason each tier above it is missing. Inputs are what each refresh already knows; no new ssh probe.
  • none: never synced, or the last cycle failed (the ssh error is the reason).
  • liveness: a live descriptor. inject: a live descriptor with a messagingSocketPath on a POSIX absolute path. attach: a live descriptor naming a tmux pane with a valid pid. launch: never available in this build.
• getRemoteHostProfile(alias) on the indexer; annotateRemoteAttachable puts the profile on the project and, on a host whose last refresh failed, remoteAttachable: false with remoteAttachBlocked / remoteStopBlocked on its sessions.
• Sidebar: the host dot's tooltip lists the tier and the missing tiers with reasons; the Stop button is disabled and the row/badge titles carry the reason where attach/stop would fail at the click. The shared stop control returns early with the reason (also covers the grid card).
• Docs: docs/remote-hosts.md, .ai/contexts/session-cache.md, CHANGELOG.

Decisions

• The tiers are independent requirements, so the reported tier is the highest available one, not the highest contiguous one (a tmux session without a messaging socket reports attach).
• A tier needing a live session reads as missing on an idle host ("nothing to read it from").
• An unreadable ~/.claude/projects fails the whole cycle, so it is not told apart from an unreachable host.
• The new-session button was already disabled for every remote host; unchanged.

Not done

• No probe (multiplexer installed with no session in it, inotifywait), no inject affordance (design(remote): choose the injection channel for remote sessions #219), no launch tier.

Tests

• test/remote-host-profile.test.js (12), test/remote-index.test.js (profile follows cycles), test/annotate-remote-attachable-local-status.test.js (blocked/healthy host), test/dom-sidebar-remote-host-tier.test.js (5, jsdom), test/stop-session-ui.test.js.
• Each new test file failed before the code (missing module / assertion). Mutations, each turning a test red: POSIX path check removed, attach rule swapped for the socket rule, && !hostBlocked dropped in annotateRemoteAttachable, Stop disabled removed.
• Not mutation-covered: the early return in confirmAndStopSession (app.js), which has no test harness.
• Related remote suites: 406 tests, 0 failures. Pre-commit task check passed.

Refs #218

[devsuitup]

Reviewing c02dbdf (adversarial review in progress).

[devsuitup]

Adversarial review at c02dbdf: no blocker; changes needed. (1) A single transient poll failure sets lastError and blocks Stop and attach, though both run their own ssh and the last descriptors are kept on failure — Stop will stay attemptable and attach block only after consecutive failures. (2) Error transitions do not re-render (notify fires only when folders changed), so the blocked state can outlive the outage; #395 adds that notify, so this PR will be rebased on it. Checked and holding: tier = highest available, attach requirement matches remoteAttachAdapter.supports, reasons set through title properties (no HTML), 54 targeted tests pass, lint 0 errors.

[devsuitup]

Re-review at 672875f: no blocker, no major. Stop is no longer gated by a poll failure; attach blocks only after 3 consecutive failures (per-host count, reset on success, !(n >= 3) so undefined/NaN never block; tests 1/2/3/9). No dead remoteStopBlocked/plan.blocked reference left; the tooltip still states the first failure's error. Minor, accepted: a forced Reconnect zeroes the count before trying, so after a long outage attach unblocks for two more cycles. Depends on #395 (error-transition notify); to be rebased on main once #395 merges.