Seed: skill name conflict on create is a skip, not fatal

[TheGreatAxios]

Summary

Fixes the seed-idempotency half of tonight's outage: a half-seeded
tenant re-running workbench seed hit 409 already exists on
default-skill creation and aborted the whole run, even though the
hub's own error advice was to re-run the same command. Every other
seed step (grants, workflow assets, deployments, catalog rows) already
treated a name-conflict 409 as "already done" — plantDefaultSkills
was the one holdout.

• plantDefaultSkills (packages/hub-client/src/seed.ts) now skips on
  a 409 from the create call, matching the pattern every other step
  uses.
• New test reproduces the exact error message from the live incident
  and fails against the pre-fix code.
• Docs: docs/seed-reconciliation.md gains a section for default
  skills, plus a section explaining why publishCorbitsToolsRegistry
  can no longer strand a dangling package-registry asset going
  forward (the freshness check already runs before the asset is
  created), and why re-running workbench seed repairs a tenant whose
  asset survives from before that ordering shipped — a repo with no
  commits answers an empty tarball list, so a re-run republishes
  everything and writes the first commit.

Not in this PR

Filed as follow-ups on CL-6575:

• Actually running workbench seed against the live broken tenant
  (tnt_4799af5615d7fa0eb995c224bc99253a) to confirm the repo
  directory gets created — I don't have admin credentials for the
  running dev stack.
• Translating SessionLaunchError's raw message into something a
  person reads as true, rather than "Myra didn't reply" — needs its
  own investigation into where launch failures actually surface in
  the product.
• A regression test locking in the freshness-check-before-asset-create
  ordering structurally (currently only confirmed by reading git
  history).

Test plan

• bun test in packages/hub-client — 164 pass
• tsc --noEmit in packages/hub-client — clean
• eslint on touched files — clean
• bun test in packages/tool-registry-publish — 61 pass (unchanged)
• bun test in packages/cli — 27 pass (unchanged)
• New test fails against the pre-fix code with the exact production error message

CL-6575