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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Hi @@copilot. Thank you for your interest in helping to improve the Azure Developer CLI experience and for your contribution. We've noticed that there hasn't been recent engagement on this pull request. If this is still an active work stream, please let us know by pushing some changes or leaving a comment. Otherwise, we'll close this out in 7 days.