config,server: add defaultProfile to select a profile at startup

[sousekd]

Closes #992

Profiles can only be activated at runtime through the UI or the API, so a restart or a configuration reload always lands on "no profile". Deployments that run unattended have no way to express which profile they want.

This adds an optional top-level defaultProfile naming a key under profiles:. It seeds the active profile when the server is built, so it applies on startup and on every configuration reload.

profiles:
  coding:
    pins:
      llm-code: "gpt-oss-120b"

defaultProfile: "coding"

Changes:

• validate defaultProfile against the configured profiles at load time, as the last step of validateProfiles
• seed Server.activeProfile from the config in New(), so the profile is active for the very first request and /v1/models reflects its pins
• document the setting in config-schema.json, config.example.yaml, and docs/configuration.md
• tests for the load path and for the seeded server

Behaviour notes:

• omitting the setting or leaving it empty keeps the current behaviour, so existing configurations are unaffected
• an unknown name fails the load with defaultProfile references unknown profile "..."
• the runtime selection is still not persisted; switching in the UI lasts until the next restart or reload, and "None" remains selectable

The diff in server.go is one added struct field plus gofmt realignment.

AI disclosure: implemented by Opus.

[coderabbitai]

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[greptile-apps]

Greptile Summary

Adds an optional validated defaultProfile setting that activates a named profile when the server starts or reloads.

• Validates that the configured default names an existing profile.
• Seeds the server’s active profile during construction.
• Documents the setting in the schema and configuration examples.
• Adds configuration-loading and server-initialization tests.

Confidence Score: 5/5

The PR appears safe to merge with no actionable defects identified.

Production startup and reload paths load validated configurations before constructing a new server, and the resulting profile-reset behavior matches the explicitly documented contract.

Important Files Changed

Filename	Overview
internal/config/config.go	Adds the YAML-backed DefaultProfile configuration field.
internal/config/load.go	Rejects nonempty default-profile values that do not identify a configured profile.
internal/server/server.go	Initializes each newly constructed server with the validated default profile, including servers created during reload.
internal/config/profile_test.go	Covers parsing a default profile and rejecting an unknown profile name.
internal/server/server_test.go	Verifies that server construction seeds the active profile.
config-schema.json	Documents the new optional string setting and its empty default.
config.example.yaml	Adds a valid default-profile example and describes reload behavior.
docs/configuration.md	Documents default-profile startup and reload semantics.

_{Reviews (1): Last reviewed commit: "config,server: add defaultProfile to sel..." | Re-trigger Greptile}

[mostlygeek]

This would be better as hooks.on_startup.profile rather than introduce a new top level config key.