fix: change AZD_SKIP_FIRST_RUN to opt-in (first-run experience off by default)

[Copilot]

Addresses #9258: the first-run tool setup experience was prompting users unexpectedly. Changes the semantics so the experience is off by default and requires explicit opt-in via AZD_SKIP_FIRST_RUN=false.

Behavior change

AZD_SKIP_FIRST_RUN value	Before	After
unset	first-run runs	first-run skipped (default_disabled)
false	first-run runs	first-run runs
true	first-run skipped	first-run skipped

Background update checks keep the old semantics (unset = run); the asymmetry is intentional and documented.

Changes

• tool_first_run.go — shouldSkip() now returns skipReasonDefaultDisabled when AZD_SKIP_FIRST_RUN is unset or unparsable; added skipReasonDefaultDisabled = "default_disabled" constant; removed AZD_SKIP_FIRST_RUN=true from the in-experience help message (it no longer applies)
• tool_update_check.go — added comment explaining the deliberate asymmetry with first-run
• internal/tracing/fields/fields.go — added default_disabled to the documented example values for tool.firstrun.skip_reason
• docs/environment-variables.md — updated AZD_SKIP_FIRST_RUN description to reflect new opt-in semantics
• tool_first_run_test.go — tests now opt in via t.Setenv(envKeySkipFirstRun, "false"); unset-state setups use t.Setenv("") + os.Unsetenv (consistent with the clearCIVars pattern) rather than bare os.Unsetenv

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