refactor(engine): merge detect + platform into one deep Environment module (deepening #5)

[ycpss91255]

Summary

Architecture deepening #5 (grilling-locked): merge lib/detect.sh +
lib/platform.sh into one deep lib/environment.sh (438 lines, under the 800
rule). Internally layered: private _environment_probe_* (I/O) ->
_environment_classify (form_factor logic) behind a small interface ->
environment_snapshot() (full {os,arch,cpu,gpu,...,form_factor} in one call)

• environment_field(). Backward-compat aliases kept (detect_environment,
  detect_get_field, platform_classify, platform_export_env); the dispatcher
  fetches the snapshot once; the runner reads gpu via environment_field.
  detect --json output is byte-identical (golden test) so the ADR-0019-adjacent
  TUI/agent contract holds. Adds Environment to CONTEXT.md.

Test plan

• test-unit: 3983 pass, 0 fail (worktree)
• test-integration: pass
• coverage AC-17: 80.81% (sharded merge); env 85.3% / dispatcher 82.8% /
  runner 93.9%
• lint clean (no new shellcheck disables; platform_export_env takes a
  required arg to satisfy SC2119/2120 without a disable)
• detect --json golden output unchanged
• rebased onto current main; no conflicts
• CI re-runs the full gate

Deepening candidate #5. .agents/memory untouched.

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