feat: implement agent socket api, client and cli

[SasSwart]

closes: #10352
closes: coder/internal#1094
closes: coder/internal#1095

In this pull request, we enable a new set of experimental cli commands grouped under coder exp sync.
These commands allow any process acting within a coder workspace to inform the coder agent of its requirements and execution progress. The coder agent will then relay this information to other processes that have subscribed.

These commands are:

# Check if this feature is enabled in your environment 
coder exp sync ping

# express that your unit depends on another
coder exp sync want <unit> <dependency_unit> 

# express that your unit intends to start a portion of the script that requires 
# other units to have completed first. This command blocks until all dependencies have been met
coder exp sync start <unit> 

# express that your unit has completes its work, allowing dependent units to begin their execution
coder exp sync complete <unit>

Example:

In order to automatically run claude code in a new workspace, it must first have a git repository cloned. The scripts responsible for cloning the repository and for running claude code would coordinate in the following way:

# Script A: Claude code

# Inform the agent that the claude script wants the git script.
# That is, the git script must have completed before the claude script can begin its execution
coder exp sync want claude git

# Inform the agent that we would now like to begin execution of claude.
# This command will block until the git script (and any other defined dependencies)
# have completed
coder exp sync start claude

# Now we run claude code and any other commands we need
claude ...

# Once our script has completed, we inform the agent, so that any scripts that depend on this one
# may begin their execution

coder exp sync complete claude

# Script B: Git

# Because the git script does not have any dependencies, we can simply inform the agent that we 
# intend to start
coder exp sync start git

git clone ssh://git@github.com/coder/coder

# Once the repository have been cloned, we inform the agent that this script is complete, so that
# scripts that depend on it may begin their execution.
coder exp sync complete git

Notes:

• Unit names (ie. claude and git) given as input to the sync commands are arbitrary strings. You do not have to conform to specific identifiers. We recommend naming your scripts descriptively, but succinctly.
• Scripts unit names should be well documented. Other scripts will need to know the names you've chosen in order to depend on yours.

[mafredri]

Looking good, some minor feedback inline, mainly re: lib design.

[mafredri]

Suggestion: client.go

[SasSwart]

@mafredri your remaining concerns are all related to the agent socket client. Can we deal with them in #20718? It deals primarily with this case. The client code here is already modified in that PR and we can include these suggestions.

[mafredri]

One minor nit about test placement, otherwise good to go 👍🏻

[mafredri]

This test belongs IMO in agent/agent_test.go.

[mafredri]

This could be shorter to make it look nicer (follow-up PR if you want).