feat(cache): resolve remote cache configuration

[wan9chi]

Motivation

Each planned execution needs a resolved remote cache endpoint and access mode, so execution can read from or upload to the remote cache without working out the settings itself. Resolving them during planning also makes them visible in plan snapshots.

Summary

• The workspace root's cache: { remote: { url } } sets the endpoint. A task opts out with cache: { remote: false }.
• --remote-cache=off|read|read-write and VP_REMOTE_CACHE select access, and the flag overrides the env. VP_REMOTE_CACHE_URL overrides the configured endpoint. Without either, access is read when an endpoint is configured and off otherwise. Empty values count as unset.
• Each vp run level resolves its own settings from the envs it sees. A level's --remote-cache reaches nested vp run through VP_REMOTE_CACHE, and a prefix such as VP_REMOTE_CACHE=off vp run build applies only to that command.
• Disabling local caching also disables remote access.
• The two env vars reach task processes and follow env and untrackedEnv like other variables. By default they pass through untracked with other VP_* variables, so runs with different remote access share cache entries.
• The endpoint is carried as a string and validated when remote caching uses it.

[github-actions]

fspy benchmark

linux

dynamic/launch             change  +0.22%  [ -6.53% ..  +6.47%]  overhead  +291.41%
dynamic/access             change  -0.31%  [ -2.87% ..  +2.58%]  overhead   +14.31%
dynamic/access-relative    change  +0.20%  [ -5.93% ..  +4.54%]  overhead   +60.83%
dynamic/access-contended   change  -2.46%  [-12.38% ..  +4.67%]  overhead   +15.63%
static/launch              change  -0.08%  [ -7.73% ..  +7.06%]  overhead  +727.11%
static/access              change  -0.07%  [ -9.02% .. +14.26%]  overhead  +813.43%
static/access-relative     change  -0.75%  [ -3.83% ..  +2.67%]  overhead +1175.75%
static/access-contended    change  +0.45%  [ -4.46% ..  +6.20%]  overhead +1994.32%

macos

dynamic/launch             change  -0.11%  [ -3.98% ..  +3.93%]  overhead  +218.65%
dynamic/access             change  +0.67%  [ -3.11% ..  +6.00%]  overhead    +2.78%
dynamic/access-relative    change  -0.68%  [ -4.57% ..  +3.02%]  overhead  +278.69%
dynamic/access-contended   change  +1.49%  [ -4.58% ..  +8.88%]  overhead    +2.19%

windows

dynamic/launch             change  +1.93%  [-15.21% .. +20.57%]  overhead   +26.50%
dynamic/access             change  +2.68%  [ -7.61% .. +27.21%]  overhead    +6.29%
dynamic/access-relative    change  +0.17%  [-21.38% .. +18.05%]  overhead    +2.95%
dynamic/access-contended   change  -2.25%  [-19.17% ..  +8.66%]  overhead    -0.09%