fix(lifecycle_emails): map experiment action_taken to friendly text

[mastermanas805]

Summary

Same bug class as worker#73 (make_permanent_endpoint): the worker email template was reading params["action_taken"] from the dashboard's /experiments/converted call and inlining the raw snake_case identifier straight into the user-facing body.

Users would receive:

Thanks for trying the new instanode experience. We noticed you checkout_started — nice.

Experiment: upgrade_button (B)

— exposing the dashboard's internal action constant + the experiments registry id as if they were English phrases.

What changed

• New friendlyExperimentAction() helper (mirrors friendlyMakePermanentSource) maps every dashboard/instanode-web emit value to a user-facing phrase. Unknown values → empty string → template clause drops entirely.
• New friendlyExperimentName() helper does the same for {{ .Experiment }}.
• renderExperimentConversion routes both fields through the helpers.

Mappings shipped:

field	enum	friendly text
action_taken	checkout_started	started the checkout flow
action_taken	overview_upgrade_clicked	clicked Upgrade on the dashboard
experiment	upgrade_button	the Upgrade button rollout
experiment	onboarding_v2	the new onboarding flow

Regression guards (CLAUDE.md rule 18 — registry-iterating)

• TestRenderExperimentConversion_ActionIsFriendly — enumerates every action value emitted by dashboard/src/components/UpgradeButton.tsx, dashboard/src/pages/OverviewPage.tsx, and the instanode-web siblings. A future emitter that adds a value without a friendly mapping fails this test before the broken copy reaches a real inbox.
• TestRenderExperimentConversion_UnknownActionDropsClause — safety net: a brand-new unmapped action drops the "We noticed you X — nice." clause entirely rather than leaking the raw enum.
• TestRenderExperimentConversion_ExperimentNameIsFriendly — same shape for the experiment-id field.

Coverage block

Symptom:        "We noticed you checkout_started — nice." in email
Enumeration:    grep -rn '"action_taken"' api/ worker/
                grep -rn 'action:\s*'"'"'[a-z_]' dashboard/src instanode-web/src
Sites found:    2 emit values (checkout_started, overview_upgrade_clicked) across 4 call sites
Sites touched:  2 (both mapped in friendlyExperimentAction)
Coverage test:  TestRenderExperimentConversion_ActionIsFriendly + ExperimentNameIsFriendly + UnknownActionDropsClause
Live verified:  awaiting user verification — emit path requires a real /experiments/converted call from a logged-in dashboard session. Renderer-level test green:
                  ok  	instant.dev/worker/internal/jobs	1.349s
                make gate green locally.

Test plan

• make gate green locally
• go test ./internal/jobs/ -run 'TestRenderExperimentConversion' → PASS (3/3)
• go test ./internal/jobs/ -run 'TestLifecycleEmail|TestEveryEmailKindHasAGoRenderer' → still green (regression unchanged)
• CI green
• Post-merge: trigger a real /experiments/converted from the dashboard UpgradeButton, confirm the resulting email body says "started the checkout flow" not "checkout_started"