chore: sync bounty board data

[sl33ty]

Publishes the four bounties to the board. Identical content to #218, recommitted so it can actually merge.

Why this exists rather than #218

sync-bounties.yml commits as github-actions[bot], using github.token because secrets.BOUNTY_SYNC_TOKEN is not set on this repo. That author fails both gates on the base branch:

• verification/cla-signed fails, since the bot has never signed the CLA.
• Merging is blocked by the verified-signature requirement, and the bot's commit is unsigned.

So the workflow runs green, produces a correct diff, and opens a PR that can never be merged. This is the same class of problem as the missing issues: read permission fixed in #208: the job succeeds while the outcome does not.

This commit carries the same generated file, authored by a CLA-covered maintainer and GPG-signed, so both gates pass.

Contents

Four bounty-labelled issues, regenerated into src/data/bounties.generated.json:

Bucket	Bounty	Reward
growth	Walk into Arrow from every direction (#214)	1,920 ARROW
operations	Put live Discord events on the community calls page (#215)	1,600 ARROW
operations	AIP-009 Accelerator (#216)	1,440 ARROW
design	Design the purchase flow on top of the store prototype (#217)	2,250 ARROW

Follow-up

The workflow still needs fixing so this is not manual every time. Add a BOUNTY_SYNC_TOKEN secret owned by a CLA-signed maintainer and have the commit step use it, rather than falling back to github.token. Tracked separately; this PR only unblocks the board.