#142 docs(insta): scrub provider branding from compute copy

Public-facing cleanup: the skill made unqualified claims that compute runs on Fly — https://<app>.fly.dev as the service URL shape, "managed Fly-backed data services", "managed DBs are Fly apps", "Fly issues the cert". Agents repeat these verbatim, which is a large part of why users believe InstaCloud still runs on Fly.

This rewrites them to plane-neutral copy:

• service URL: "its public service URL (insta --agent services list prints it)"
• "Fly-backed" → "legacy-plane" / "legacy compute plane" throughout SKILL.md, cli-reference.md, references/deploy.md, references/operate.md
• "Fly's remote builder" → "the legacy plane's remote builder"

Deliberately kept, because removing them would break users on legacy services:

• flyctl named once per doc as the auto-installed legacy build tool (it visibly installs during a legacy deploy)
• fly as the literal manifest wire value (provider: fly | insta-compute) that agents must match

Migration-from-Fly guides (references/migrate/fly.md etc.) are intentional mentions and untouched.

🤖 Generated with Claude Code

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Summary by cubic

Rewrites the InstaCloud compute docs to stop claiming compute runs on Fly, so agents stop repeating provider branding back to users.

• Replaces https://<app>.fly.dev service URLs with the output of insta --agent service list.
• Renames "Fly-backed" to "legacy plane" throughout SKILL.md, cli-reference.md, deploy.md, and operate.md.
• Keeps the two operational literals: flyctl as the auto-installed legacy build tool and fly as the manifest's provider wire value, which agents must match.
• Leaves the migration-from-Fly guides untouched.