Reframe kickoff as the initiative's brain-dump

[kmcginnes]

Description

Refines the Kickoff definition in docs/agents/issue-tracker.md. A parallel session read the prior "thin trigger / does not own the scoping" framing and mistook the kickoff for an empty placeholder.

The kickoff is the opposite of empty: it holds the raw, unscoped brain-dump for an initiative — hypotheses, references to libraries/prior art, suspected blockers, open questions. It has content; it's just missing the chart. A later Wayfinder session reads that dump as its seed input and grills from there to produce the map.

• Kickoff entry rewritten to lead with the brain-dump content and drop "thin trigger" (now in Avoid).
• Wayfinder entry: "off a Kickoff placeholder" → "seeded by a Kickoff's brain-dump".
• Optionality unchanged: skip the kickoff and the same thoughts go straight into a live Wayfinder session.

Validation

Docs-only. `pnpm check:format` passes.

Check List

• I confirm that my contribution is made under the terms of the Apache 2.0 license.
• I have verified `pnpm checks` passes with no errors.
• I have verified `pnpm test` passes with no failures.
• I have covered new added functionality with unit tests if necessary.
• I have updated documentation if necessary.