feat: add Ascii Box remote execution environments

[zozo123]

Summary

Adds an internal/adapter/boxenv placement backend for Ascii Box using Mecatl's existing PlacementProvider / PlacementReattacher execution-environment seam.

• provisions a fresh Box for the default placement and persists it as EnvironmentRef{Kind:"box", ID:<box-id>, Revision:"box-api-v1"}
• reattaches the exact persisted Box and resumes stopped/archived environments without silently creating a replacement
• binds Box filesystem operations and Shell to the same /workspace namespace
• implements Read/Stat/Glob/Grep and versioned Create/Replace with Mecatl's read-before-write CAS semantics for a live workspace handle
• implements the optional namespace capability used by ListDir/Remove/Move/Copy
• uses Box's v1 HTTP API directly (no SDK dependency)
• defaults Box creation to noEnv=true, keeping account credentials and secrets out of agent sandboxes
• preserves the existing no-filesystem attenuation
• validates HTTPS endpoints, bounds API responses/errors, confines workspace paths, and never persists/logs the Box API key

Verification

Added an offline httptest fake Box API covering:

• default placement creation with noEnv=true
• exact stopped-Box reattach/resume with no replacement creation
• workspace/runner affinity
• file create/read/version conflict behavior
• Glob/Grep
• path-escape rejection
• API {ok:false} failures and fs.ErrNotExist mapping

The GitHub workflows for this fork currently report action_required (fork workflow approval), not a test failure.

A live Box API smoke was attempted from the current execution environment with the supplied credential, but its outbound DNS cannot resolve ascii.dev; the credential is not included in this branch, PR, or CI.

Follow-ups

This PR intentionally does not invent remote worktree/fork semantics. Box-native fork/merge can be added once it is mapped explicitly to Mecatl's remote placement/delegation contract rather than pretending to be a local worktree selector.

[jhrozek]

looks like this was superseded by #1604 reopen if it's not the case