Add CodeMash conference data

[sodalone]

/claim #10

Adds CodeMash to the conference dataset using public CodeMash sources and the issue's conservative SEA guidance.

Sources:

• CodeMash speaker benefits / CFP details: https://codemash.org/call-for-speakers-announcement/
• CodeMash 2026 CFP announcement: https://codemash.org/codemash-2026-call-for-speakers/
• CodeMash overview / organizer page: https://codemash.org/About/

Values added:

• Sign SEA?: No, because the issue says to assume no SEA willingness unless the organizer confirms otherwise.
• Free Ticket?: Yes, because speaker benefits include a four day pass.
• Reimburse Expenses?: Partial, because speaker benefits include a housing credit rather than full travel reimbursement.
• Compensate Speakers?: Workshops, because PreCompiler workshop sessions receive an honorarium.

Validation:

• Checked the README table rows keep the expected column count.

[sodalone]

Validation note for review:

PR diff scope: README.md only
GitHub mergeability: clean

The added CodeMash entry includes the conference name, canonical website, location, and event focus in the same README table style as the existing data tracker entries.