template: improve proposal form from RFP-004 dogfood feedback

[fryorcraken]

Summary

Improvements to the RFP proposal submission form (.github/ISSUE_TEMPLATE/proposal.yml), driven by dogfooding the process with an internal team on RFP-004 (#40). A competent, contracted team still hit several template rough edges, so these are template fixes, not submitter faults.

Changes

• New required "Requirement Coverage" field. The biggest gap was that nothing connected a proposal back to the RFP it targets. This field asks the submitter to confirm coverage of each hard requirement and to acknowledge the RFP's Out of Scope and Platform Dependencies. In RFP-004 — Privacy-Preserving Decentralized Exchange #40 this would have surfaced (a) a deliverable that the merged RFP-004 now lists as Out of Scope, and (b) several required deliverables (CLI, IDL, doc packets, benchmarking) that went unmentioned.
• Per-milestone payouts enforced; budget reconciled. The budget field claimed to equal "the sum of milestone payouts," but the milestone format never required payouts, so RFP-004 — Privacy-Preserving Decentralized Exchange #40's $10k total was unverifiable. Now each milestone must state a Payout: line. An audit milestone may leave its payout blank when an auditing team still needs to be contracted, in which case it is excluded from the budget sum until set.
• Project Summary tightened to ~2 paragraphs on why the RFP matters for the Logos ecosystem and community. The prior open-ended prompt drew a "not sure what to put here" answer in RFP-004 — Privacy-Preserving Decentralized Exchange #40.
• Relevant Experience reworked to ask for links to specific shipped projects or products (a repository, a live app, a published package), not personal/org profiles or resumes. The prior "shipped products / prior work / resumes" wording was ambiguous and RFP-004 — Privacy-Preserving Decentralized Exchange #40 pasted bare @handles.
• Replaced the vague Post-Delivery Plan field with a focused "Support Window" field: how long the team commits to fixing bugs after final delivery. The old field drew a one-word answer in RFP-004 — Privacy-Preserving Decentralized Exchange #40.
• "Before you start" note prompting submitters to read the RFP's Scope / Out of Scope / Platform Dependencies before submitting.

Test plan

• proposal.yml parses as valid YAML and as a GitHub issue form (unique field IDs, all interactive fields have IDs)
• Render check: open the form via "New issue" once merged to confirm field order and markdown render

Context: #40