fix(sessions): stop the opening prompt reporting kubernetes-context as unset

[sourcehawk]

Description

Every investigation's opening prompt reported kubernetes-context: <unset> even when preflight had seeded a context from the operator's cluster pick, so the agent believed no context was bound while the k8s MCP was already pointed at the cluster (surfaced in session 1940ad81… as a scan for a cluster it already had). The seeded name lived in Investigation.ActiveContext and <sessionDir>/active-context but was never copied into sessions.Options, so prompts.Env.Context stayed empty. This PR plumbs it through on both the fresh-start and rehydrate paths. The teleport/kubeconfig auth guidance in the system prompt still tells the agent to switch_context before its first k8s call regardless of what the prompt names; that wording is left as-is.

Changes

• Investigation.ActiveContext now flows into sessions.Options.Cluster from manager.go (fresh start) and rehydrate.go (resume after launcher restart), which also makes it available to namespace_derivation as cluster for the first time.
• Session.Start delegates prompt rendering to a new startPrompt() that sets prompts.Env.Context, so the rendered Environment block is testable without spawning claude.

Testing

Two new tests in internal/sessions: a seeded Cluster renders kubernetes-context: <name> and an empty one still renders <unset>; the first was watched failing on the missing method before the implementation. make lint is clean and make test-go (race) passes in full. Frontend untouched. To poke at it manually, start an investigation with a cluster selected and check the first ## Environment block in the transcript.

🤖 Generated with Claude Code

https://claude.ai/code/session_01RWH96oteD5VB4R9togLq5V

[Copilot]

🟢 Approval recommended

The change is narrowly scoped, is exercised by new unit tests, and correctly wires the seeded context through to the prompt renderer without altering runtime behavior beyond the intended prompt output.

Pull request overview

Fixes the investigation opening prompt’s ## Environment block so it reports the kubeconfig context the launcher already seeded (instead of always showing kubernetes-context: <unset>), aligning the agent’s perceived cluster binding with the actual k8s MCP state.

Changes:

• Plumbs Investigation.ActiveContext into sessions.Options.Cluster on both fresh start and rehydrate paths.
• Refactors Session.Start to delegate opening-prompt rendering to a new startPrompt() helper that sets prompts.Env.Context.
• Adds focused unit tests asserting seeded vs unset rendering for kubernetes-context.

File summaries

File	Description
internal/sessions/session.go	Uses opts.Cluster to set prompts.Env.Context (driving kubernetes-context rendering) and refactors Start to call startPrompt().
internal/sessions/session_test.go	Adds tests verifying startPrompt() emits the seeded kube context and preserves <unset> when empty.
internal/server/rehydrate.go	Passes inv.ActiveContext into sessions.Options.Cluster during session rebuild after restart.
internal/server/manager.go	Passes i.ActiveContext into sessions.Options.Cluster when starting a new investigation session.

Review details

• Files reviewed: 4/4 changed files
• Comments generated: 0
• Review effort level: Lite

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