docs(jira): align setup guide with implemented behavior (#552)

[ayushtr-aws]

Closes #552.

Docs-only. Aligns docs/guides/JIRA_SETUP_GUIDE.md with what a live setup walkthrough of the integration actually requires (dev-stack deployment, 2026-07-07/08). No behavior described here is new — the guide now matches the code as shipped.

Changes

• Prerequisites — state the stack must be deployed at a version including the Jira integration (feat(jira): Jira Cloud integration -- parity with Linear (#288) #302); pre-feat(jira): Jira Cloud integration -- parity with Linear (#288) #302 stacks have no Jira resources and need a sync + redeploy first.
• Step 2 (OAuth) — new note for the "Waiting for browser callback…" stall (wrong browser profile, closed consent tab, port 8080 conflict) and that Ctrl-C + re-run is safe/idempotent. Mirrored as a Troubleshooting entry.
• Step 4 (map) — point at setup's ready-to-paste "Next steps" map command; document cloudId recovery (https://<site>.atlassian.net/_edge/tenant_info or the workspace registry) since it's not in the Jira UI.
• Step 5 (identity linking) — the guide said "populate the user-mapping row manually" without schema or command. Now includes the exact aws dynamodb put-item with the row shape jira-link.ts writes (jira_identity = <cloudId>#<accountId>, platform_user_id, linked_at, status: active, link_method), how to find the Atlassian accountId and Cognito sub, a cross-link to CLI: implement bgagent jira invite-user — issuing half of Jira identity linking #553 (bgagent jira invite-user), and a note that the processor's "Run bgagent jira link <code>" comment is a dead end until CLI: implement bgagent jira invite-user — issuing half of Jira identity linking #553 lands.
• Troubleshooting — new "401 when calling the Jira API directly after setup" entry: the ~1 h token life is expected, refresh happens Lambda-side on the next webhook, and manual refresh is hazardous (Atlassian rotates the refresh token per use; the write-back must preserve webhook_signing_secret).
• Diagram — POST /jira/webhook → POST /v1/jira/webhook (API GW stage prefix).

Deliberately not touched: the map-time repo-onboarding prerequisite — #374 (fix for #369) adds that line; avoiding a conflict.

Verification

• mise run build green (includes //docs:sync mirror regeneration + docs link-check); the Starlight mirror docs/src/content/docs/using/Jira-setup-guide.md is regenerated in the same commit.
• The Step-5 put-item shape was validated against a live deployment: a manually-written row with exactly these fields attributes label-triggered tasks correctly (verified via bgagent list and the processor logs).

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