bug: LOG_UTC is an unnamespaced environment variable

[codeforester]

Problem

CliFormatter reads LOG_UTC from the process environment
(lib/python/base_cli/logging.py:193):

self.use_utc = use_utc if use_utc is not None else os.environ.get("LOG_UTC") == "1"

LOG_UTC is a generic, unnamespaced name. Every other base-cli-specific environment variable is
prefixed, and the only unprefixed names the package reads otherwise are recognized cross-tool
standards (NO_COLOR, XDG_CACHE_HOME, XDG_CONFIG_HOME, LOCALAPPDATA, APPDATA, SHELL,
COMSPEC):

$ grep -rhoE "BASE_CLI_[A-Z_]+" lib/python/base_cli/ | sort -u
BASE_CLI_CACHE_DIR   BASE_CLI_COLOR   BASE_CLI_CONFIG_DIR   BASE_CLI_DISPLAY_COMMAND

LOG_UTC is not a standard. Any unrelated tool, CI image, or shell profile that exports LOG_UTC=1
silently changes the timestamp format of every base-cli CLI on that machine, and the log timestamps
are part of the retained diagnostic bundle. It is also undocumented: it appears in no file under
docs/.

Verified evidence

Reviewed 2026-09-30 at a58ec109349fa3f3d03eae5b0de078b39ea361a2.

os.environ["LOG_UTC"] = "1"
CliFormatter().use_utc -> True

$ grep -rn "LOG_UTC" docs/
(no matches)

Proposal

1. Read BASE_CLI_LOG_UTC as the supported name.
2. Continue honouring LOG_UTC for one release with a BaseCliDeprecationWarning (the project
   already has deprecations.deprecated() and a documented policy in docs/api-stability.md), then
   remove it.
3. Document both the variable and the full environment surface. There is currently no single page
   listing the environment variables base-cli reads; adding one to docs/integrations.md would also
   serve the other env-var work.

Acceptance criteria

• BASE_CLI_LOG_UTC controls UTC timestamps.
• LOG_UTC still works but warns, with the removal release named.
• One documentation page lists every environment variable base-cli reads, its meaning, and its
  default.

Non-goals

• Do not change the default timestamp format or the datefmt strings.