chore: drop personal lineage + perennial entries from team git-town config

[mhotan]

Summary

.git-branches.toml is the team-shared git-town config. Today it carries entries that only mean something to one contributor's working branches:

perennials = ["mike/monitoring-testing-wip"]

[lineage]
"mike/prometheus-refactor" = "main"
"mike/controlplane-monitoring" = "mike/prometheus-refactor"

Those refs don't exist for anyone else and just clutter the shared config. Personal lineage + perennial bookkeeping belongs in each contributor's local .git/config (which git town set-parent and git config git-town.perennial-branches write to).

Changes

• perennials = []
• [lineage] empty

Test plan

• git town config still parses; lineage shows whatever's set in local .git/config per contributor.
• No effect on CI, builds, or any chart output.

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